Index: clang-tidy/ClangTidyOptions.h
===================================================================
--- clang-tidy/ClangTidyOptions.h
+++ clang-tidy/ClangTidyOptions.h
@@ -87,6 +87,17 @@
   /// \brief Key-value mapping used to store check-specific options.
   OptionMap CheckOptions;
 
+  /// \brief The source of the option.
+  //
+  /// clang-tidy has 3 types of the sources (from low to high):
+  ///    * clang-tidy binary
+  ///    * '-config' commandline option or a specific configuration file
+  ///    * '-checks' commandline option
+  std::string Source;
+
+  /// \brief Stores all options that have higher priority than current one.
+  std::vector<ClangTidyOptions> HigherPriorityOptions;
+
   typedef std::vector<std::string> ArgList;
 
   /// \brief Add extra compilation arguments to the end of the list.
Index: clang-tidy/ClangTidyOptions.cpp
===================================================================
--- clang-tidy/ClangTidyOptions.cpp
+++ clang-tidy/ClangTidyOptions.cpp
@@ -120,6 +120,11 @@
 ClangTidyOptions::mergeWith(const ClangTidyOptions &Other) const {
   ClangTidyOptions Result = *this;
 
+  Result.HigherPriorityOptions.push_back(Other);
+  Result.HigherPriorityOptions.insert(Result.HigherPriorityOptions.end(),
+      Other.HigherPriorityOptions.begin(),
+      Other.HigherPriorityOptions.end());
+
   // Merge comma-separated glob lists by appending the new value after a comma.
   if (Other.Checks)
     Result.Checks =
@@ -255,7 +260,7 @@
                      << ParsedOptions.getError().message() << "\n";
       continue;
     }
-
+    ParsedOptions->Source = ConfigFile.c_str();
     return DefaultOptionsProvider::getOptions(Directory)
         .mergeWith(*ParsedOptions)
         .mergeWith(OverrideOptions);
Index: clang-tidy/tool/ClangTidyMain.cpp
===================================================================
--- clang-tidy/tool/ClangTidyMain.cpp
+++ clang-tidy/tool/ClangTidyMain.cpp
@@ -128,6 +128,12 @@
 )"),
                                 cl::init(false), cl::cat(ClangTidyCategory));
 
+static cl::opt<bool> ExplainConfig("explain-config", cl::desc(R"(
+for each enabled check explains, where it is enabled, i.e. in clang-tidy binary,
+command line or a specific configuration file.
+)"),
+                                   cl::init(false), cl::cat(ClangTidyCategory));
+
 static cl::opt<std::string> Config("config", cl::desc(R"(
 Specifies a configuration in YAML/JSON format:
   -config="{Checks: '*',
@@ -179,6 +185,12 @@
 namespace clang {
 namespace tidy {
 
+static constexpr char CheckSourceTypeDefaultBinary[] = "clang-tidy binary";
+static constexpr char CheckSourceTypeCheckCommandLineOption[] =
+    "command-line option '-checks'";
+static constexpr char CheckSourceTypeConfigCommandLineOption[] =
+    "command-line option '-config'";
+
 static void printStats(const ClangTidyStats &Stats) {
   if (Stats.errorsIgnored()) {
     llvm::errs() << "Suppressed " << Stats.errorsIgnored() << " warnings (";
@@ -256,6 +268,7 @@
 
   ClangTidyOptions DefaultOptions;
   DefaultOptions.Checks = DefaultChecks;
+  DefaultOptions.Source = CheckSourceTypeDefaultBinary;
   DefaultOptions.WarningsAsErrors = "";
   DefaultOptions.HeaderFilterRegex = HeaderFilter;
   DefaultOptions.SystemHeaders = SystemHeaders;
@@ -266,6 +279,7 @@
     DefaultOptions.User = llvm::sys::Process::GetEnv("USERNAME");
 
   ClangTidyOptions OverrideOptions;
+  OverrideOptions.Source = CheckSourceTypeCheckCommandLineOption;
   if (Checks.getNumOccurrences() > 0)
     OverrideOptions.Checks = Checks;
   if (WarningsAsErrors.getNumOccurrences() > 0)
@@ -280,6 +294,7 @@
   if (!Config.empty()) {
     if (llvm::ErrorOr<ClangTidyOptions> ParsedConfig =
             parseConfiguration(Config)) {
+      ParsedConfig->Source = CheckSourceTypeConfigCommandLineOption;
       return llvm::make_unique<DefaultOptionsProvider>(
           GlobalOptions, ClangTidyOptions::getDefaults()
                              .mergeWith(DefaultOptions)
@@ -311,6 +326,28 @@
   ClangTidyOptions EffectiveOptions = OptionsProvider->getOptions(FileName);
   std::vector<std::string> EnabledChecks = getCheckNames(EffectiveOptions);
 
+  if (ExplainConfig) {
+    // FIXME: Figure out a more elegant way to show other ClangTidyOptions'
+    // fields like ExtraArg.
+    for (const std::string &Check : EnabledChecks) {
+      bool IsFindMatch = false;
+      for (auto It = EffectiveOptions.HigherPriorityOptions.rbegin();
+           It != EffectiveOptions.HigherPriorityOptions.rend(); ++It) {
+        if (It->Checks && GlobList(*(It->Checks)).contains(Check)) {
+          llvm::outs() << "'" << Check << "' is enabled in the " << It->Source
+                       << ".\n";
+          IsFindMatch = true;
+          break;
+        }
+      }
+      if (!IsFindMatch) {
+        llvm::outs() << "'" << Check << "' is enabled in the "
+                     << EffectiveOptions.Source << ".\n";
+      }
+    }
+    return 0;
+  }
+
   if (ListChecks) {
     llvm::outs() << "Enabled checks:";
     for (auto CheckName : EnabledChecks)
Index: test/clang-tidy/Inputs/explain-config/.clang-tidy
===================================================================
--- /dev/null
+++ test/clang-tidy/Inputs/explain-config/.clang-tidy
@@ -0,0 +1 @@
+Checks: '-*,modernize-use-nullptr'
Index: test/clang-tidy/explain-checks.cpp
===================================================================
--- /dev/null
+++ test/clang-tidy/explain-checks.cpp
@@ -0,0 +1,14 @@
+// RUN: clang-tidy -checks=-*,modernize-use-nullptr -explain-config | FileCheck --check-prefix=CHECK-MESSAGE1 %s
+// RUN: clang-tidy -config="{Checks: '-*,modernize-use-nullptr'}" -explain-config | FileCheck --check-prefix=CHECK-MESSAGE2 %s
+// RUN: clang-tidy -checks=modernize-use-nullptr -config="{Checks: '-*,modernize-use-nullptr'}" -explain-config | FileCheck --check-prefix=CHECK-MESSAGE3 %s
+// RUN: clang-tidy -checks=modernize-use-nullptr -config="{Checks: '-*,-modernize-use-nullptr'}" %S/Inputs/explain-config/a.cc -explain-config | FileCheck --check-prefix=CHECK-MESSAGE4 %s
+// RUN: clang-tidy -checks=modernize-use-nullptr -config="{Checks: '-*,modernize-*'}" -explain-config | FileCheck --check-prefix=CHECK-MESSAGE5 %s
+// RUN: clang-tidy -checks=modernize-use-nullptr -explain-config | FileCheck --check-prefix=CHECK-MESSAGE6 %s
+// RUN: clang-tidy -explain-config %S/Inputs/explain-config/a.cc | grep "'modernize-use-nullptr' is enabled in the %S/Inputs/explain-config/.clang-tidy."
+
+// CHECK-MESSAGE1: 'modernize-use-nullptr' is enabled in the command-line option '-checks'.
+// CHECK-MESSAGE2: 'modernize-use-nullptr' is enabled in the command-line option '-config'.
+// CHECK-MESSAGE3: 'modernize-use-nullptr' is enabled in the command-line option '-checks'.
+// CHECK-MESSAGE4: 'modernize-use-nullptr' is enabled in the command-line option '-checks'.
+// CHECK-MESSAGE5: 'modernize-use-nullptr' is enabled in the command-line option '-checks'.
+// CHECK-MESSAGE6: 'clang-analyzer-unix.API' is enabled in the clang-tidy binary.