diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 927cc050e20..08f2654d4d2 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -314,6 +314,21 @@ module ts {
                 if (!s && nameNotFoundMessage) {
                     error(errorLocation, nameNotFoundMessage, nameArg);
                 }
+                if (s && s.flags & SymbolFlags.BlockScoped) {
+                    var declaration = forEach(s.declarations, d => d.flags & NodeFlags.BlockScoped ? d : undefined);
+                    Debug.assert(declaration, "Bock-scoped variable declaration is undefined");
+                    var declarationSourceFile = getSourceFileOfNode(declaration);
+                    var referenceSourceFile = getSourceFileOfNode(errorLocation);
+                    if (declarationSourceFile === referenceSourceFile && declaration.pos > errorLocation.pos) {
+                        error(errorLocation, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, identifierToString(declaration.name));
+                    }
+                    else if (compilerOptions.out) {
+                        var sourceFiles = program.getSourceFiles();
+                        if (sourceFiles.indexOf(referenceSourceFile) < sourceFiles.indexOf(declarationSourceFile)) {
+                            error(errorLocation, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, identifierToString(declaration.name));
+                        }
+                    }
+                }
                 return s;
             }
 
diff --git a/src/compiler/diagnosticInformationMap.generated.ts b/src/compiler/diagnosticInformationMap.generated.ts
index 84e10cfec58..f2a1f6abc03 100644
--- a/src/compiler/diagnosticInformationMap.generated.ts
+++ b/src/compiler/diagnosticInformationMap.generated.ts
@@ -269,6 +269,7 @@ module ts {
         Property_0_is_protected_and_only_accessible_within_class_1_and_its_subclasses: { code: 2445, category: DiagnosticCategory.Error, key: "Property '{0}' is protected and only accessible within class '{1}' and its subclasses." },
         Property_0_is_protected_and_only_accessible_through_an_instance_of_class_1: { code: 2446, category: DiagnosticCategory.Error, key: "Property '{0}' is protected and only accessible through an instance of class '{1}'." },
         The_0_operator_is_not_allowed_for_boolean_types_Consider_using_1_instead: { code: 2447, category: DiagnosticCategory.Error, key: "The '{0}' operator is not allowed for boolean types. Consider using '{1}' instead." },
+        Block_scoped_variable_0_used_before_its_declaration: { code: 2448, category: DiagnosticCategory.Error, key: "Block-scoped variable '{0}' used before its declaration." },
         Import_declaration_0_is_using_private_name_1: { code: 4000, category: DiagnosticCategory.Error, key: "Import declaration '{0}' is using private name '{1}'." },
         Type_parameter_0_of_exported_class_has_or_is_using_name_1_from_private_module_2: { code: 4001, category: DiagnosticCategory.Error, key: "Type parameter '{0}' of exported class has or is using name '{1}' from private module '{2}'." },
         Type_parameter_0_of_exported_class_has_or_is_using_private_name_1: { code: 4002, category: DiagnosticCategory.Error, key: "Type parameter '{0}' of exported class has or is using private name '{1}'." },
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index fa433e0e772..dddd3ddb3d2 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -451,27 +451,27 @@
         "category": "Error",
         "code":  1151
     },
-	"'var', 'let' or 'const' expected.": {
+    "'var', 'let' or 'const' expected.": {
         "category": "Error",
         "code": 1152
     },
-	 "'let' variable declarations are only available when targeting ECMAScript 6 and higher.": {
+    "'let' variable declarations are only available when targeting ECMAScript 6 and higher.": {
         "category": "Error",
         "code": 1153
     },
-	 "'const' variable declarations are only available when targeting ECMAScript 6 and higher.": {
+    "'const' variable declarations are only available when targeting ECMAScript 6 and higher.": {
         "category": "Error",
         "code": 1154
     },
-	"const must be intialized.": {
+    "const must be intialized.": {
         "category": "Error",
         "code": 1155
     },
-	"const must be declared inside a block.": {
+    "const must be declared inside a block.": {
         "category": "Error",
         "code": 1156
     },
-	"let must be declared inside a block.": {
+    "let must be declared inside a block.": {
         "category": "Error",
         "code": 1157
     },
@@ -1068,6 +1068,10 @@
         "category": "Error",
         "code": 2447
     },
+    "Block-scoped variable '{0}' used before its declaration.": {
+        "category": "Error",
+        "code": 2448
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
diff --git a/tests/baselines/reference/constDeclarations-useBeforeDefinition.errors.txt b/tests/baselines/reference/constDeclarations-useBeforeDefinition.errors.txt
new file mode 100644
index 00000000000..97cf58c6e9d
--- /dev/null
+++ b/tests/baselines/reference/constDeclarations-useBeforeDefinition.errors.txt
@@ -0,0 +1,21 @@
+tests/cases/compiler/constDeclarations-useBeforeDefinition.ts(3,5): error TS2448: Block-scoped variable 'c1' used before its declaration.
+tests/cases/compiler/constDeclarations-useBeforeDefinition.ts(9,5): error TS2448: Block-scoped variable 'v1' used before its declaration.
+
+
+==== tests/cases/compiler/constDeclarations-useBeforeDefinition.ts (2 errors) ====
+    
+    {
+        c1;
+        ~~
+!!! error TS2448: Block-scoped variable 'c1' used before its declaration.
+        const c1 = 0;
+    }
+    
+    var v1;
+    {
+        v1;
+        ~~
+!!! error TS2448: Block-scoped variable 'v1' used before its declaration.
+        const v1 = 0;
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/constDeclarations-useBeforeDefinition.js b/tests/baselines/reference/constDeclarations-useBeforeDefinition.js
new file mode 100644
index 00000000000..4fe0e64f275
--- /dev/null
+++ b/tests/baselines/reference/constDeclarations-useBeforeDefinition.js
@@ -0,0 +1,24 @@
+//// [constDeclarations-useBeforeDefinition.ts]
+
+{
+    c1;
+    const c1 = 0;
+}
+
+var v1;
+{
+    v1;
+    const v1 = 0;
+}
+
+
+//// [constDeclarations-useBeforeDefinition.js]
+{
+    c1;
+    const c1 = 0;
+}
+var v1;
+{
+    v1;
+    const v1 = 0;
+}
diff --git a/tests/baselines/reference/constDeclarations-useBeforeDefinition2.errors.txt b/tests/baselines/reference/constDeclarations-useBeforeDefinition2.errors.txt
new file mode 100644
index 00000000000..a4d28f51878
--- /dev/null
+++ b/tests/baselines/reference/constDeclarations-useBeforeDefinition2.errors.txt
@@ -0,0 +1,11 @@
+tests/cases/compiler/file1.ts(2,1): error TS2448: Block-scoped variable 'c' used before its declaration.
+
+
+==== tests/cases/compiler/file1.ts (1 errors) ====
+    
+    c;
+    ~
+!!! error TS2448: Block-scoped variable 'c' used before its declaration.
+    
+==== tests/cases/compiler/file2.ts (0 errors) ====
+    const c = 0;
\ No newline at end of file
diff --git a/tests/baselines/reference/constDeclarations-useBeforeDefinition2.js b/tests/baselines/reference/constDeclarations-useBeforeDefinition2.js
new file mode 100644
index 00000000000..7d1f4cf5725
--- /dev/null
+++ b/tests/baselines/reference/constDeclarations-useBeforeDefinition2.js
@@ -0,0 +1,12 @@
+//// [tests/cases/compiler/constDeclarations-useBeforeDefinition2.ts] ////
+
+//// [file1.ts]
+
+c;
+
+//// [file2.ts]
+const c = 0;
+
+//// [out.js]
+c;
+const c = 0;
diff --git a/tests/baselines/reference/letDeclarations-useBeforeDefinition.errors.txt b/tests/baselines/reference/letDeclarations-useBeforeDefinition.errors.txt
new file mode 100644
index 00000000000..74c354f4ff2
--- /dev/null
+++ b/tests/baselines/reference/letDeclarations-useBeforeDefinition.errors.txt
@@ -0,0 +1,21 @@
+tests/cases/compiler/letDeclarations-useBeforeDefinition.ts(3,5): error TS2448: Block-scoped variable 'l1' used before its declaration.
+tests/cases/compiler/letDeclarations-useBeforeDefinition.ts(9,5): error TS2448: Block-scoped variable 'v1' used before its declaration.
+
+
+==== tests/cases/compiler/letDeclarations-useBeforeDefinition.ts (2 errors) ====
+    
+    {
+        l1;
+        ~~
+!!! error TS2448: Block-scoped variable 'l1' used before its declaration.
+        let l1;
+    }
+    
+    var v1;
+    {
+        v1;
+        ~~
+!!! error TS2448: Block-scoped variable 'v1' used before its declaration.
+        let v1 = 0;
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/letDeclarations-useBeforeDefinition.js b/tests/baselines/reference/letDeclarations-useBeforeDefinition.js
new file mode 100644
index 00000000000..be1f9a127f4
--- /dev/null
+++ b/tests/baselines/reference/letDeclarations-useBeforeDefinition.js
@@ -0,0 +1,24 @@
+//// [letDeclarations-useBeforeDefinition.ts]
+
+{
+    l1;
+    let l1;
+}
+
+var v1;
+{
+    v1;
+    let v1 = 0;
+}
+
+
+//// [letDeclarations-useBeforeDefinition.js]
+{
+    l1;
+    let l1;
+}
+var v1;
+{
+    v1;
+    let v1 = 0;
+}
diff --git a/tests/baselines/reference/letDeclarations-useBeforeDefinition2.errors.txt b/tests/baselines/reference/letDeclarations-useBeforeDefinition2.errors.txt
new file mode 100644
index 00000000000..5b8633312d9
--- /dev/null
+++ b/tests/baselines/reference/letDeclarations-useBeforeDefinition2.errors.txt
@@ -0,0 +1,11 @@
+tests/cases/compiler/file1.ts(2,1): error TS2448: Block-scoped variable 'l' used before its declaration.
+
+
+==== tests/cases/compiler/file1.ts (1 errors) ====
+    
+    l;
+    ~
+!!! error TS2448: Block-scoped variable 'l' used before its declaration.
+    
+==== tests/cases/compiler/file2.ts (0 errors) ====
+    const l = 0;
\ No newline at end of file
diff --git a/tests/baselines/reference/letDeclarations-useBeforeDefinition2.js b/tests/baselines/reference/letDeclarations-useBeforeDefinition2.js
new file mode 100644
index 00000000000..35560edd1c8
--- /dev/null
+++ b/tests/baselines/reference/letDeclarations-useBeforeDefinition2.js
@@ -0,0 +1,12 @@
+//// [tests/cases/compiler/letDeclarations-useBeforeDefinition2.ts] ////
+
+//// [file1.ts]
+
+l;
+
+//// [file2.ts]
+const l = 0;
+
+//// [out.js]
+l;
+const l = 0;
diff --git a/tests/baselines/reference/letDeclarations3.errors.txt b/tests/baselines/reference/letDeclarations3.errors.txt
new file mode 100644
index 00000000000..8c4d925ce13
--- /dev/null
+++ b/tests/baselines/reference/letDeclarations3.errors.txt
@@ -0,0 +1,18 @@
+tests/cases/compiler/letDeclarations3.ts(3,5): error TS2300: Duplicate identifier 'l1'.
+tests/cases/compiler/letDeclarations3.ts(3,9): error TS2300: Duplicate identifier 'l1'.
+tests/cases/compiler/letDeclarations3.ts(3,13): error TS2300: Duplicate identifier 'l1'.
+
+
+==== tests/cases/compiler/letDeclarations3.ts (3 errors) ====
+    
+    // Duplicate variables
+    let l1, l1, l1;
+        ~~
+!!! error TS2300: Duplicate identifier 'l1'.
+            ~~
+!!! error TS2300: Duplicate identifier 'l1'.
+                ~~
+!!! error TS2300: Duplicate identifier 'l1'.
+    
+    // unexpected 'let'
+    let l2, let, l3;
\ No newline at end of file
diff --git a/tests/cases/compiler/constDeclarations-useBeforeDefinition.ts b/tests/cases/compiler/constDeclarations-useBeforeDefinition.ts
new file mode 100644
index 00000000000..ec793fda484
--- /dev/null
+++ b/tests/cases/compiler/constDeclarations-useBeforeDefinition.ts
@@ -0,0 +1,12 @@
+// @target: ES6
+
+{
+    c1;
+    const c1 = 0;
+}
+
+var v1;
+{
+    v1;
+    const v1 = 0;
+}
diff --git a/tests/cases/compiler/constDeclarations-useBeforeDefinition2.ts b/tests/cases/compiler/constDeclarations-useBeforeDefinition2.ts
new file mode 100644
index 00000000000..c34bb4012bf
--- /dev/null
+++ b/tests/cases/compiler/constDeclarations-useBeforeDefinition2.ts
@@ -0,0 +1,8 @@
+// @target: ES6
+// @out: out.js
+
+// @Filename: file1.ts
+c;
+
+// @Filename: file2.ts
+const c = 0;
\ No newline at end of file
diff --git a/tests/cases/compiler/letDeclarations-useBeforeDefinition.ts b/tests/cases/compiler/letDeclarations-useBeforeDefinition.ts
new file mode 100644
index 00000000000..671762d4a45
--- /dev/null
+++ b/tests/cases/compiler/letDeclarations-useBeforeDefinition.ts
@@ -0,0 +1,12 @@
+// @target: ES6
+
+{
+    l1;
+    let l1;
+}
+
+var v1;
+{
+    v1;
+    let v1 = 0;
+}
diff --git a/tests/cases/compiler/letDeclarations-useBeforeDefinition2.ts b/tests/cases/compiler/letDeclarations-useBeforeDefinition2.ts
new file mode 100644
index 00000000000..a6cbc729b94
--- /dev/null
+++ b/tests/cases/compiler/letDeclarations-useBeforeDefinition2.ts
@@ -0,0 +1,8 @@
+// @target: ES6
+// @out: out.js
+
+// @Filename: file1.ts
+l;
+
+// @Filename: file2.ts
+const l = 0;
\ No newline at end of file