Index: clang-tidy/ClangTidyOptions.h
===================================================================
--- clang-tidy/ClangTidyOptions.h
+++ clang-tidy/ClangTidyOptions.h
@@ -59,6 +59,10 @@
   /// of this instance overridden by the fields of \p Other that have a value.
   ClangTidyOptions mergeWith(const ClangTidyOptions &Other) const;
 
+  /// \brief Inserts checks' source in CheckSource which is used to record
+  /// relationship between each check and its source.
+  void insertCheckSource(llvm::StringRef Checks, llvm::StringRef Source);
+
   /// \brief Checks filter.
   llvm::Optional<std::string> Checks;
 
@@ -87,6 +91,10 @@
   /// \brief Key-value mapping used to store check-specific options.
   OptionMap CheckOptions;
 
+  /// \brief Key-value mapping used to store relationship between check filters
+  /// and the source file where the filters come from.
+  OptionMap CheckSources;
+
   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
@@ -100,6 +100,13 @@
 
 namespace clang {
 namespace tidy {
+namespace {
+SmallVector<StringRef, 5> SplitChecks(StringRef Checks) {
+  SmallVector<StringRef, 5> Result;
+  Checks.split(Result, ',');
+  return Result;
+}
+} // namespace
 
 ClangTidyOptions ClangTidyOptions::getDefaults() {
   ClangTidyOptions Options;
@@ -121,10 +128,18 @@
   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;
+
+    SmallVector<StringRef, 5> Checks = SplitChecks(*Other.Checks);
+    for (StringRef Check: Checks) {
+      const auto it = Other.CheckSources.find(Check);
+      if (it != Other.CheckSources.end())
+        Result.CheckSources[Check] = it->second;
+    }
+  }
   if (Other.WarningsAsErrors)
     Result.WarningsAsErrors =
         (Result.WarningsAsErrors && !Result.WarningsAsErrors->empty()
@@ -151,6 +166,13 @@
   return Result;
 }
 
+void ClangTidyOptions::insertCheckSource(StringRef Checks, StringRef Source) {
+  SmallVector<StringRef, 5> SC = SplitChecks(Checks);
+  for (StringRef Check: SC) {
+    CheckSources[Check] = Source;
+  }
+}
+
 FileOptionsProvider::FileOptionsProvider(
     const ClangTidyGlobalOptions &GlobalOptions,
     const ClangTidyOptions &DefaultOptions,
@@ -255,7 +277,10 @@
                      << ParsedOptions.getError().message() << "\n";
       continue;
     }
-
+    if (ParsedOptions->Checks) {
+      ParsedOptions->insertCheckSource(*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 +262,7 @@
 
   ClangTidyOptions DefaultOptions;
   DefaultOptions.Checks = DefaultChecks;
+  DefaultOptions.insertCheckSource(DefaultChecks, "clang-tidy binary");
   DefaultOptions.WarningsAsErrors = "";
   DefaultOptions.HeaderFilterRegex = HeaderFilter;
   DefaultOptions.SystemHeaders = SystemHeaders;
@@ -266,8 +273,10 @@
     DefaultOptions.User = llvm::sys::Process::GetEnv("USERNAME");
 
   ClangTidyOptions OverrideOptions;
-  if (Checks.getNumOccurrences() > 0)
+  if (Checks.getNumOccurrences() > 0) {
     OverrideOptions.Checks = Checks;
+    OverrideOptions.insertCheckSource(Checks, "command-line option '-checks'");
+  }
   if (WarningsAsErrors.getNumOccurrences() > 0)
     OverrideOptions.WarningsAsErrors = WarningsAsErrors;
   if (HeaderFilter.getNumOccurrences() > 0)
@@ -280,6 +289,10 @@
   if (!Config.empty()) {
     if (llvm::ErrorOr<ClangTidyOptions> ParsedConfig =
             parseConfiguration(Config)) {
+      if (ParsedConfig->Checks) {
+        ParsedConfig->insertCheckSource(*ParsedConfig->Checks,
+            "command-line option '-config'");
+      }
       return llvm::make_unique<DefaultOptionsProvider>(
           GlobalOptions, ClangTidyOptions::getDefaults()
                              .mergeWith(DefaultOptions)
@@ -311,6 +324,19 @@
   ClangTidyOptions EffectiveOptions = OptionsProvider->getOptions(FileName);
   std::vector<std::string> EnabledChecks = getCheckNames(EffectiveOptions);
 
+  if (ExplainConfig) {
+    for (const std::string& Check : EnabledChecks) {
+      for (const ClangTidyOptions::StringPair &CheckSource:
+           EffectiveOptions.CheckSources) {
+        if (GlobList(CheckSource.first).contains(Check)) {
+          llvm::outs() << "'" << Check << "' is enabled in the "
+                       << CheckSource.second << ".\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 +1,12 @@
+// 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 -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'.