diff --git a/src/Type/Definition/BooleanType.php b/src/Type/Definition/BooleanType.php
index 3446094..262b274 100644
--- a/src/Type/Definition/BooleanType.php
+++ b/src/Type/Definition/BooleanType.php
@@ -7,6 +7,8 @@ class BooleanType extends ScalarType
 {
     public $name = Type::BOOLEAN;
 
+    public $description = 'The `Boolean` scalar type represents `true` or `false`.';
+
     public function serialize($value)
     {
         return !!$value;
diff --git a/src/Type/Definition/FloatType.php b/src/Type/Definition/FloatType.php
index da58b47..d40bf6b 100644
--- a/src/Type/Definition/FloatType.php
+++ b/src/Type/Definition/FloatType.php
@@ -8,6 +8,11 @@ class FloatType extends ScalarType
 {
     public $name = Type::FLOAT;
 
+    public $description =
+        'The `Float` scalar type represents signed double-precision fractional ' .
+        'values as specified by ' .
+        '[IEEE 754](http://en.wikipedia.org/wiki/IEEE_floating_point). ';
+
     public function serialize($value)
     {
         return $this->coerceFloat($value);
diff --git a/src/Type/Definition/IDType.php b/src/Type/Definition/IDType.php
index e6f1deb..ead6ea4 100644
--- a/src/Type/Definition/IDType.php
+++ b/src/Type/Definition/IDType.php
@@ -8,6 +8,13 @@ class IDType extends ScalarType
 {
     public $name = 'ID';
 
+    public $description =
+        'The `ID` scalar type represents a unique identifier, often used to ' .
+        'refetch an object or as key for a cache. The ID type appears in a JSON ' .
+        'response as a String; however, it is not intended to be human-readable. ' .
+        'When expected as an input type, any string (such as `"4"`) or integer ' .
+        '(such as `4`) input value will be accepted as an ID.';
+
     public function serialize($value)
     {
         return (string) $value;
diff --git a/src/Type/Definition/IntType.php b/src/Type/Definition/IntType.php
index 8370e53..f21ae1c 100644
--- a/src/Type/Definition/IntType.php
+++ b/src/Type/Definition/IntType.php
@@ -6,8 +6,20 @@ use GraphQL\Language\AST\Value;
 
 class IntType extends ScalarType
 {
+    // As per the GraphQL Spec, Integers are only treated as valid when a valid
+    // 32-bit signed integer, providing the broadest support across platforms.
+    //
+    // n.b. JavaScript's integers are safe between -(2^53 - 1) and 2^53 - 1 because
+    // they are internally represented as IEEE 754 doubles.
+    const MAX_INT = 2147483647;
+    const MIN_INT = -2147483648;
+
     public $name = Type::INT;
 
+    public $description =
+        'The `Int` scalar type represents non-fractional signed whole numeric ' .
+        'values. Int can represent values between -(2^31) and 2^31 - 1. ';
+
     public function serialize($value)
     {
         return $this->coerceInt($value);
@@ -23,7 +35,7 @@ class IntType extends ScalarType
         if (false === $value || true === $value) {
             return (int) $value;
         }
-        if (is_numeric($value) && $value <= PHP_INT_MAX && $value >= -1 * PHP_INT_MAX) {
+        if (is_numeric($value) && $value <= self::MAX_INT && $value >= self::MIN_INT) {
             return (int) $value;
         }
         return null;
@@ -33,7 +45,7 @@ class IntType extends ScalarType
     {
         if ($ast instanceof IntValue) {
             $val = (int) $ast->value;
-            if ($ast->value === (string) $val) {
+            if ($ast->value === (string) $val && self::MIN_INT <= $val && $val <= self::MAX_INT) {
                 return $val;
             }
         }
diff --git a/src/Type/Definition/StringType.php b/src/Type/Definition/StringType.php
index cccea49..9d51969 100644
--- a/src/Type/Definition/StringType.php
+++ b/src/Type/Definition/StringType.php
@@ -7,6 +7,11 @@ class StringType extends ScalarType
 {
     public $name = Type::STRING;
 
+    public $description =
+        'The `String` scalar type represents textual data, represented as UTF-8 ' .
+        'character sequences. The String type is most often used by GraphQL to ' .
+        'represent free-form human-readable text.';
+
     public function serialize($value)
     {
         return $this->parseValue($value);