diff --git a/search/CMakeLists.txt b/search/CMakeLists.txt
index 5fe22c5437..640173cdbb 100644
--- a/search/CMakeLists.txt
+++ b/search/CMakeLists.txt
@@ -12,9 +12,13 @@ set(
   base/text_index/header.hpp
   base/text_index/mem.cpp
   base/text_index/mem.hpp
+  base/text_index/merger.cpp
+  base/text_index/merger.hpp  
+  base/text_index/postings.hpp
   base/text_index/reader.hpp
   base/text_index/text_index.cpp
   base/text_index/text_index.hpp
+  base/text_index/utils.hpp
   bookmarks/data.cpp
   bookmarks/data.hpp
   bookmarks/processor.cpp
diff --git a/search/base/text_index/mem.hpp b/search/base/text_index/mem.hpp
index 26059e3f1f..367becbe14 100644
--- a/search/base/text_index/mem.hpp
+++ b/search/base/text_index/mem.hpp
@@ -2,14 +2,15 @@
 
 #include "search/base/text_index/dictionary.hpp"
 #include "search/base/text_index/header.hpp"
+#include "search/base/text_index/postings.hpp"
 #include "search/base/text_index/text_index.hpp"
+#include "search/base/text_index/utils.hpp"
 
 #include "coding/reader.hpp"
 #include "coding/varint.hpp"
 #include "coding/write_to_sink.hpp"
 
 #include "base/assert.hpp"
-#include "base/checked_cast.hpp"
 #include "base/string_utils.hpp"
 
 #include <algorithm>
@@ -84,11 +85,34 @@ public:
   }
 
 private:
-  template <typename Sink>
-  static uint32_t RelativePos(Sink & sink, uint64_t startPos)
+  class MemPostingsFetcher : public PostingsFetcher
   {
-    return ::base::checked_cast<uint32_t>(sink.Pos() - startPos);
-  }
+  public:
+    MemPostingsFetcher(std::map<Token, std::vector<Posting>> const & postingsByToken)
+    {
+      // todo(@m) An unnecessary copy?
+      m_postings.reserve(postingsByToken.size());
+      for (auto const & entry : postingsByToken)
+        m_postings.emplace_back(entry.second);
+    }
+
+    // PostingsFetcher overrides:
+    bool GetPostingsForNextToken(std::vector<uint32_t> & postings)
+    {
+      CHECK_LESS_OR_EQUAL(m_tokenId, m_postings.size(), ());
+      if (m_tokenId == m_postings.size())
+        return false;
+      postings.swap(m_postings[m_tokenId++]);
+      return true;
+    }
+
+  private:
+    std::vector<std::vector<uint32_t>> m_postings;
+    // Index of the next token to be processed. The
+    // copy of the postings list in |m_postings| is not guaranteed
+    // to be valid after it's been processed.
+    size_t m_tokenId = 0;
+  };
 
   void SortPostings();
 
@@ -110,41 +134,8 @@ private:
   template <typename Sink>
   void SerializePostingsLists(Sink & sink, TextIndexHeader & header, uint64_t startPos) const
   {
-    header.m_postingsStartsOffset = RelativePos(sink, startPos);
-    // An uint32_t for each 32-bit offset and an uint32_t for the dummy entry at the end.
-    WriteZeroesToSink(sink, sizeof(uint32_t) * (header.m_numTokens + 1));
-
-    header.m_postingsListsOffset = RelativePos(sink, startPos);
-
-    std::vector<uint32_t> postingsStarts;
-    postingsStarts.reserve(header.m_numTokens);
-    for (auto const & entry : m_postingsByToken)
-    {
-      auto const & postings = entry.second;
-
-      postingsStarts.emplace_back(RelativePos(sink, startPos));
-
-      uint32_t last = 0;
-      for (auto const p : postings)
-      {
-        CHECK(last == 0 || last < p, (last, p));
-        uint32_t const delta = p - last;
-        WriteVarUint(sink, delta);
-        last = p;
-      }
-    }
-    // One more for convenience.
-    postingsStarts.emplace_back(RelativePos(sink, startPos));
-
-    {
-      uint64_t const savedPos = sink.Pos();
-      sink.Seek(startPos + header.m_postingsStartsOffset);
-      for (uint32_t const s : postingsStarts)
-        WriteToSink(sink, s);
-
-      CHECK_EQUAL(sink.Pos(), startPos + header.m_postingsListsOffset, ());
-      sink.Seek(savedPos);
-    }
+    MemPostingsFetcher fetcher(m_postingsByToken);
+    WritePostings(sink, startPos, header, fetcher);
   }
 
   template <typename Source>
diff --git a/search/base/text_index/merger.cpp b/search/base/text_index/merger.cpp
new file mode 100644
index 0000000000..7ea8425475
--- /dev/null
+++ b/search/base/text_index/merger.cpp
@@ -0,0 +1,107 @@
+#include "search/base/text_index/merger.hpp"
+
+#include "search/base/text_index/dictionary.hpp"
+#include "search/base/text_index/header.hpp"
+#include "search/base/text_index/postings.hpp"
+
+#include "coding/file_writer.hpp"
+#include "coding/varint.hpp"
+#include "coding/write_to_sink.hpp"
+
+#include "base/assert.hpp"
+#include "base/logging.hpp"
+#include "base/stl_add.hpp"
+#include "base/stl_helpers.hpp"
+
+#include <algorithm>
+#include <utility>
+#include <vector>
+
+using namespace std;
+
+namespace
+{
+using namespace search::base;
+
+class MergedPostingsListFetcher : public PostingsFetcher
+{
+public:
+  MergedPostingsListFetcher(TextIndexDictionary const & dict, TextIndexReader const & index1,
+                            TextIndexReader const & index2)
+    : m_dict(dict), m_index1(index1), m_index2(index2)
+  {
+  }
+
+  // PostingsFetcher overrides:
+  bool GetPostingsForNextToken(std::vector<uint32_t> & postings)
+  {
+    postings.clear();
+
+    auto const & tokens = m_dict.GetTokens();
+    CHECK_LESS_OR_EQUAL(m_tokenId, tokens.size(), ());
+    if (m_tokenId == tokens.size())
+      return false;
+
+    m_index1.ForEachPosting(tokens[m_tokenId], MakeBackInsertFunctor(postings));
+    m_index2.ForEachPosting(tokens[m_tokenId], MakeBackInsertFunctor(postings));
+    my::SortUnique(postings);
+    ++m_tokenId;
+    return true;
+  }
+
+private:
+  TextIndexDictionary const & m_dict;
+  TextIndexReader const & m_index1;
+  TextIndexReader const & m_index2;
+  // Index of the next token from |m_dict| to be processed.
+  size_t m_tokenId = 0;
+};
+
+TextIndexDictionary MergeDictionaries(TextIndexDictionary const & dict1,
+                                      TextIndexDictionary const & dict2)
+{
+  vector<Token> commonTokens = dict1.GetTokens();
+  for (auto const & token : dict2.GetTokens())
+  {
+    size_t dummy;
+    if (!dict1.GetTokenId(token, dummy))
+      commonTokens.emplace_back(token);
+  }
+
+  sort(commonTokens.begin(), commonTokens.end());
+  TextIndexDictionary dict;
+  dict.SetTokens(move(commonTokens));
+  return dict;
+}
+}  // namespace
+
+namespace search
+{
+namespace base
+{
+// static
+void TextIndexMerger::Merge(TextIndexReader const & index1, TextIndexReader const & index2,
+                            FileWriter & sink)
+{
+  TextIndexDictionary const dict =
+      MergeDictionaries(index1.GetDictionary(), index2.GetDictionary());
+
+  TextIndexHeader header;
+
+  uint64_t const startPos = sink.Pos();
+  // Will be filled in later.
+  header.Serialize(sink);
+
+  dict.Serialize(sink, header, startPos);
+
+  MergedPostingsListFetcher fetcher(dict, index1, index2);
+  WritePostings(sink, startPos, header, fetcher);
+
+  // Fill in the header.
+  uint64_t const finishPos = sink.Pos();
+  sink.Seek(startPos);
+  header.Serialize(sink);
+  sink.Seek(finishPos);
+}
+}  // namespace base
+}  // namespace search
diff --git a/search/base/text_index/merger.hpp b/search/base/text_index/merger.hpp
new file mode 100644
index 0000000000..5e8fb3b3ac
--- /dev/null
+++ b/search/base/text_index/merger.hpp
@@ -0,0 +1,30 @@
+#pragma once
+
+#include "search/base/text_index/reader.hpp"
+
+class FileWriter;
+
+namespace search
+{
+namespace base
+{
+// Merges two on-disk text indexes and writes them to a new one.
+class TextIndexMerger
+{
+public:
+  // The merging process is as follows.
+  // 1. Dictionaries from both indexes are read into memory, merged
+  //    and written to disk.
+  // 2. One uint32_t per entry is reserved in memory to calculate the
+  //    offsets of the postings lists.
+  // 3. One token at a time, all postings for the token are read from
+  //    both indexes into memory, unified and written to disk.
+  // 4. The offsets are written to disk.
+  //
+  // Note that the dictionary and offsets are kept in memory during the whole
+  // merging process.
+  static void Merge(TextIndexReader const & index1, TextIndexReader const & index2,
+                    FileWriter & sink);
+};
+}  // namespace base
+}  // namespace search
diff --git a/search/base/text_index/postings.hpp b/search/base/text_index/postings.hpp
new file mode 100644
index 0000000000..ccfc32ab25
--- /dev/null
+++ b/search/base/text_index/postings.hpp
@@ -0,0 +1,77 @@
+#pragma once
+
+#include "search/base/text_index/header.hpp"
+#include "search/base/text_index/text_index.hpp"
+#include "search/base/text_index/utils.hpp"
+
+#include "coding/varint.hpp"
+
+#include <vector>
+
+namespace search
+{
+namespace base
+{
+struct TextIndexHeader;
+
+// A helper class that fetches the postings lists for
+// one token at a time. It is assumed that the tokens
+// are enumerated in the lexicographic order.
+class PostingsFetcher
+{
+public:
+  // Returns true and fills |postings| with the postings list of the next token
+  // when there is one.
+  // Returns false if the underlying source is exhausted, i.e. there are
+  // no more tokens left.
+  virtual bool GetPostingsForNextToken(std::vector<uint32_t> & postings) = 0;
+};
+
+// Fetches the postings list one by one from |fetcher| and writes them
+// to |sink|, updating the fields in |header| that correspond to the
+// postings list.
+// |startPos| marks the start of the entire text index and is needed to compute
+// the offsets that are stored in |header|.
+template <typename Sink>
+void WritePostings(Sink & sink, uint64_t startPos, TextIndexHeader & header,
+                   PostingsFetcher & fetcher)
+{
+  header.m_postingsStartsOffset = RelativePos(sink, startPos);
+  // An uint32_t for each 32-bit offset and an uint32_t for the dummy entry at the end.
+  WriteZeroesToSink(sink, sizeof(uint32_t) * (header.m_numTokens + 1));
+
+  header.m_postingsListsOffset = RelativePos(sink, startPos);
+
+  std::vector<uint32_t> postingsStarts;
+  postingsStarts.reserve(header.m_numTokens);
+
+  // todo(@m) s/uint32_t/Posting/ ?
+  std::vector<uint32_t> postings;
+  while (fetcher.GetPostingsForNextToken(postings))
+  {
+    postingsStarts.emplace_back(RelativePos(sink, startPos));
+
+    uint32_t last = 0;
+    for (auto const p : postings)
+    {
+      CHECK(last == 0 || last < p, (last, p));
+      uint32_t const delta = p - last;
+      WriteVarUint(sink, delta);
+      last = p;
+    }
+  }
+  // One more for convenience.
+  postingsStarts.emplace_back(RelativePos(sink, startPos));
+
+  {
+    uint64_t const savedPos = sink.Pos();
+    sink.Seek(startPos + header.m_postingsStartsOffset);
+    for (uint32_t const s : postingsStarts)
+      WriteToSink(sink, s);
+
+    CHECK_EQUAL(sink.Pos(), startPos + header.m_postingsListsOffset, ());
+    sink.Seek(savedPos);
+  }
+}
+}  // namespace base
+}  // namespace search
diff --git a/search/base/text_index/reader.hpp b/search/base/text_index/reader.hpp
index 5ad39034d3..772aacd77e 100644
--- a/search/base/text_index/reader.hpp
+++ b/search/base/text_index/reader.hpp
@@ -69,6 +69,8 @@ public:
     ForEachPosting(std::move(utf8s), std::forward<Fn>(fn));
   }
 
+  TextIndexDictionary const & GetDictionary() const { return m_dictionary; }
+
 private:
   FileReader m_fileReader;
   TextIndexDictionary m_dictionary;
diff --git a/search/base/text_index/utils.hpp b/search/base/text_index/utils.hpp
new file mode 100644
index 0000000000..fe896c6c37
--- /dev/null
+++ b/search/base/text_index/utils.hpp
@@ -0,0 +1,17 @@
+#pragma once
+
+#include "base/checked_cast.hpp"
+
+#include <cstdint>
+
+namespace search
+{
+namespace base
+{
+template <typename Sink>
+static uint32_t RelativePos(Sink & sink, uint64_t startPos)
+{
+  return ::base::checked_cast<uint32_t>(sink.Pos() - startPos);
+}
+}  // namespace base
+}  // namespace search
diff --git a/search/search_tests/text_index_tests.cpp b/search/search_tests/text_index_tests.cpp
index 15cbaa70b3..b161007f14 100644
--- a/search/search_tests/text_index_tests.cpp
+++ b/search/search_tests/text_index_tests.cpp
@@ -1,6 +1,7 @@
 #include "testing/testing.hpp"
 
 #include "search/base/text_index/mem.hpp"
+#include "search/base/text_index/merger.hpp"
 #include "search/base/text_index/reader.hpp"
 #include "search/base/text_index/text_index.hpp"
 
@@ -8,6 +9,7 @@
 
 #include "platform/platform_tests_support/scoped_file.hpp"
 
+#include "coding/file_writer.hpp"
 #include "coding/reader.hpp"
 #include "coding/write_to_sink.hpp"
 #include "coding/writer.hpp"
@@ -33,6 +35,23 @@ namespace
 // Prepend several bytes to serialized indexes in order to check the relative offsets.
 size_t const kSkip = 10;
 
+search::base::MemTextIndex BuildMemTextIndex(vector<string> const & docsCollection)
+{
+  MemTextIndex memIndex;
+
+  for (size_t docId = 0; docId < docsCollection.size(); ++docId)
+  {
+    strings::SimpleTokenizer tok(docsCollection[docId], " ");
+    while (tok)
+    {
+      memIndex.AddPosting(*tok, static_cast<uint32_t>(docId));
+      ++tok;
+    }
+  }
+
+  return memIndex;
+}
+
 void Serdes(MemTextIndex & memIndex, MemTextIndex & deserializedMemIndex, vector<uint8_t> & buf)
 {
   buf.clear();
@@ -54,7 +73,7 @@ void TestForEach(Index const & index, Token const & token, vector<uint32_t> cons
 {
   vector<uint32_t> actual;
   index.ForEachPosting(token, MakeBackInsertFunctor(actual));
-  TEST_EQUAL(actual, expected, ());
+  TEST_EQUAL(actual, expected, (token));
 };
 }  // namespace
 
@@ -69,17 +88,7 @@ UNIT_TEST(TextIndex_Smoke)
       "a c",
   };
 
-  MemTextIndex memIndex;
-
-  for (size_t docId = 0; docId < docsCollection.size(); ++docId)
-  {
-    strings::SimpleTokenizer tok(docsCollection[docId], " ");
-    while (tok)
-    {
-      memIndex.AddPosting(*tok, static_cast<uint32_t>(docId));
-      ++tok;
-    }
-  }
+  auto memIndex = BuildMemTextIndex(docsCollection);
 
   vector<uint8_t> indexData;
   MemTextIndex deserializedMemIndex;
@@ -139,4 +148,60 @@ UNIT_TEST(TextIndex_UniString)
     TestForEach(index, strings::MakeUniString("ç"), {0, 1});
   }
 }
+
+UNIT_TEST(TextIndex_Merging)
+{
+  using Token = base::Token;
+
+  // todo(@m) Arrays? docsCollection[i]
+  vector<Token> const docsCollection1 = {
+      "a b c",
+      "",
+      "d",
+  };
+  vector<Token> const docsCollection2 = {
+      "",
+      "a c",
+      "e",
+  };
+
+  auto memIndex1 = BuildMemTextIndex(docsCollection1);
+  vector<uint8_t> indexData1;
+  MemTextIndex deserializedMemIndex1;
+  Serdes(memIndex1, deserializedMemIndex1, indexData1);
+
+  auto memIndex2 = BuildMemTextIndex(docsCollection2);
+  vector<uint8_t> indexData2;
+  MemTextIndex deserializedMemIndex2;
+  Serdes(memIndex2, deserializedMemIndex2, indexData2);
+
+  {
+    string contents1;
+    copy_n(indexData1.begin() + kSkip, indexData1.size() - kSkip, back_inserter(contents1));
+    ScopedFile file1("text_index_tmp1", contents1);
+    FileReader fileReader1(file1.GetFullPath());
+    TextIndexReader textIndexReader1(fileReader1);
+
+    string contents2;
+    copy_n(indexData2.begin() + kSkip, indexData2.size() - kSkip, back_inserter(contents2));
+    ScopedFile file2("text_index_tmp2", contents2);
+    FileReader fileReader2(file2.GetFullPath());
+    TextIndexReader textIndexReader2(fileReader2);
+
+    ScopedFile file3("text_index_tmp3", ScopedFile::Mode::Create);
+    {
+      FileWriter fileWriter(file3.GetFullPath());
+      TextIndexMerger::Merge(textIndexReader1, textIndexReader2, fileWriter);
+    }
+
+    FileReader fileReader3(file3.GetFullPath());
+    TextIndexReader textIndexReader3(fileReader3);
+    TestForEach(textIndexReader3, "a", {0, 1});
+    TestForEach(textIndexReader3, "b", {0});
+    TestForEach(textIndexReader3, "c", {0, 1});
+    TestForEach(textIndexReader3, "x", {});
+    TestForEach(textIndexReader3, "d", {2});
+    TestForEach(textIndexReader3, "e", {2});
+  }
+}
 }  // namespace search