Index: clang-tidy/ClangTidyOptions.h
===================================================================
--- clang-tidy/ClangTidyOptions.h
+++ clang-tidy/ClangTidyOptions.h
@@ -87,6 +87,19 @@
   /// \brief Key-value mapping used to store check-specific options.
   OptionMap CheckOptions;
 
+  enum CheckSourceType {
+    DefaultBinary,
+    ConfigCommandlineOptionOrFile,
+    ChecksCommandlineOption,
+    CheckSourceTypeEnd
+  };
+  // \brief Stores each check filter and its source for every check source type.
+  // clang-tidy has 3 types of check sources:
+  //    clang-tidy binary
+  //    '-config' commandline option or a specific configuration file
+  //    '-checks' commandline option
+  std::vector<StringPair> CheckSources[CheckSourceTypeEnd];
+
   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
@@ -121,10 +121,18 @@
   ClangTidyOptions Result = *this;
 
   // Merge comma-separated glob lists by appending the new value after a comma.
-  if (Other.Checks)
+  if (Other.Checks) {
     Result.Checks =
         (Result.Checks && !Result.Checks->empty() ? *Result.Checks + "," : "") +
         *Other.Checks;
+
+    for (auto SourceType : {ClangTidyOptions::ChecksCommandlineOption,
+                            ClangTidyOptions::ConfigCommandlineOptionOrFile,
+                            ClangTidyOptions::DefaultBinary}) {
+      for (const auto &CheckSource : Other.CheckSources[SourceType])
+        Result.CheckSources[SourceType].push_back(CheckSource);
+    }
+  }
   if (Other.WarningsAsErrors)
     Result.WarningsAsErrors =
         (Result.WarningsAsErrors && !Result.WarningsAsErrors->empty()
@@ -255,7 +263,12 @@
                      << ParsedOptions.getError().message() << "\n";
       continue;
     }
-
+    if (ParsedOptions->Checks) {
+      ParsedOptions
+          ->CheckSources[ClangTidyOptions::ConfigCommandlineOptionOrFile]
+          .push_back(ClangTidyOptions::StringPair(*ParsedOptions->Checks,
+                                                  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: '*',
@@ -256,6 +262,8 @@
 
   ClangTidyOptions DefaultOptions;
   DefaultOptions.Checks = DefaultChecks;
+  DefaultOptions.CheckSources[ClangTidyOptions::DefaultBinary].push_back(
+      ClangTidyOptions::StringPair(DefaultChecks, "clang-tidy binary"));
   DefaultOptions.WarningsAsErrors = "";
   DefaultOptions.HeaderFilterRegex = HeaderFilter;
   DefaultOptions.SystemHeaders = SystemHeaders;
@@ -266,8 +274,12 @@
     DefaultOptions.User = llvm::sys::Process::GetEnv("USERNAME");
 
   ClangTidyOptions OverrideOptions;
-  if (Checks.getNumOccurrences() > 0)
+  if (Checks.getNumOccurrences() > 0) {
     OverrideOptions.Checks = Checks;
+    OverrideOptions.CheckSources[ClangTidyOptions::ChecksCommandlineOption]
+        .push_back(ClangTidyOptions::StringPair(
+            Checks, "command-line option '-checks'"));
+  }
   if (WarningsAsErrors.getNumOccurrences() > 0)
     OverrideOptions.WarningsAsErrors = WarningsAsErrors;
   if (HeaderFilter.getNumOccurrences() > 0)
@@ -280,6 +292,12 @@
   if (!Config.empty()) {
     if (llvm::ErrorOr<ClangTidyOptions> ParsedConfig =
             parseConfiguration(Config)) {
+      if (ParsedConfig->Checks) {
+        ParsedConfig
+            ->CheckSources[ClangTidyOptions::ConfigCommandlineOptionOrFile]
+            .push_back(ClangTidyOptions::StringPair(
+                *ParsedConfig->Checks, "command-line option '-config'"));
+      }
       return llvm::make_unique<DefaultOptionsProvider>(
           GlobalOptions, ClangTidyOptions::getDefaults()
                              .mergeWith(DefaultOptions)
@@ -311,6 +329,29 @@
   ClangTidyOptions EffectiveOptions = OptionsProvider->getOptions(FileName);
   std::vector<std::string> EnabledChecks = getCheckNames(EffectiveOptions);
 
+  if (ExplainConfig) {
+    for (const std::string& Check : EnabledChecks) {
+      // Keeps the processing order of different check sources.
+      for (auto SourceType : {ClangTidyOptions::ChecksCommandlineOption,
+                              ClangTidyOptions::ConfigCommandlineOptionOrFile,
+                              ClangTidyOptions::DefaultBinary}) {
+        bool FindMatch = false;
+        for (auto It = EffectiveOptions.CheckSources[SourceType].rbegin();
+             It != EffectiveOptions.CheckSources[SourceType].rend(); ++It) {
+          if (GlobList(It->first).contains(Check)) {
+            llvm::outs() << "'" << Check << "' is enabled in the "
+                         << It->second << ".\n";
+            FindMatch = true;
+            break;
+          }
+          if (FindMatch)
+            break;
+        }
+      }
+    }
+    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 +1,12 @@
+// 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 -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'.