diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@@ -3,6 +3,8 @@
     libc.src.ctype.isalnum
     libc.src.ctype.isalpha
     libc.src.ctype.isdigit
+    libc.src.ctype.islower
+    libc.src.ctype.isupper
     
     # errno.h entrypoints
     libc.src.errno.__errno_location
diff --git a/libc/config/linux/api.td b/libc/config/linux/api.td
--- a/libc/config/linux/api.td
+++ b/libc/config/linux/api.td
@@ -91,6 +91,8 @@
     "isalnum",
     "isalpha",
     "isdigit",
+    "islower",
+    "isupper",
   ];
 }
 
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -6,6 +6,8 @@
     libc.src.ctype.isalnum
     libc.src.ctype.isalpha
     libc.src.ctype.isdigit
+    libc.src.ctype.islower
+    libc.src.ctype.isupper
 
     # errno.h entrypoints
     libc.src.errno.__errno_location
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -61,6 +61,16 @@
               RetValSpec<IntType>,
               [ArgSpec<IntType>]
           >,
+          FunctionSpec<
+              "islower",
+              RetValSpec<IntType>,
+              [ArgSpec<IntType>]
+          >,
+          FunctionSpec<
+              "isupper",
+              RetValSpec<IntType>,
+              [ArgSpec<IntType>]
+          >,
       ]
   >;
      
diff --git a/libc/src/ctype/CMakeLists.txt b/libc/src/ctype/CMakeLists.txt
--- a/libc/src/ctype/CMakeLists.txt
+++ b/libc/src/ctype/CMakeLists.txt
@@ -33,3 +33,19 @@
   DEPENDS
     .ctype_utils
 )
+
+add_entrypoint_object(
+  islower
+  SRCS
+    islower.cpp
+  HDRS
+    islower.h
+)
+
+add_entrypoint_object(
+  isupper
+  SRCS
+    isupper.cpp
+  HDRS
+    isupper.h
+)
diff --git a/libc/src/ctype/islower.h b/libc/src/ctype/islower.h
new file mode 100644
--- /dev/null
+++ b/libc/src/ctype/islower.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for islower -------------------------*-C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_CTYPE_ISLOWER_H
+#define LLVM_LIBC_SRC_CTYPE_ISLOWER_H
+
+namespace __llvm_libc {
+
+int islower(int c);
+
+} // namespace __llvm_libc
+
+#endif //  LLVM_LIBC_SRC_CTYPE_ISLOWER_H
diff --git a/libc/src/ctype/islower.cpp b/libc/src/ctype/islower.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/ctype/islower.cpp
@@ -0,0 +1,22 @@
+//===-- Implementation of islower------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/ctype/islower.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+// TODO: Currently restricted to default locale.
+// These should be extended using locale information.
+int LLVM_LIBC_ENTRYPOINT(islower)(int c) {
+  const unsigned ch = c;
+  return (ch - 'a') < 26;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/src/ctype/isupper.h b/libc/src/ctype/isupper.h
new file mode 100644
--- /dev/null
+++ b/libc/src/ctype/isupper.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for isupper -------------------------*-C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_CTYPE_ISUPPER_H
+#define LLVM_LIBC_SRC_CTYPE_ISUPPER_H
+
+namespace __llvm_libc {
+
+int isupper(int c);
+
+} // namespace __llvm_libc
+
+#endif //  LLVM_LIBC_SRC_CTYPE_ISUPPER_H
diff --git a/libc/src/ctype/isupper.cpp b/libc/src/ctype/isupper.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/ctype/isupper.cpp
@@ -0,0 +1,22 @@
+//===-- Implementation of isupper------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/ctype/isupper.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+// TODO: Currently restricted to default locale.
+// These should be extended using locale information.
+int LLVM_LIBC_ENTRYPOINT(isupper)(int c) {
+  const unsigned ch = c;
+  return (ch - 'A') < 26;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/ctype/CMakeLists.txt b/libc/test/src/ctype/CMakeLists.txt
--- a/libc/test/src/ctype/CMakeLists.txt
+++ b/libc/test/src/ctype/CMakeLists.txt
@@ -29,3 +29,23 @@
   DEPENDS
     libc.src.ctype.isdigit
 )
+
+add_libc_unittest(
+  islower
+  SUITE
+    libc_ctype_unittests
+  SRCS
+    islower_test.cpp
+  DEPENDS
+    libc.src.ctype.islower
+)
+
+add_libc_unittest(
+  isupper
+  SUITE
+    libc_ctype_unittests
+  SRCS
+    isupper_test.cpp
+  DEPENDS
+    libc.src.ctype.isupper
+)
diff --git a/libc/test/src/ctype/isalnum_test.cpp b/libc/test/src/ctype/isalnum_test.cpp
--- a/libc/test/src/ctype/isalnum_test.cpp
+++ b/libc/test/src/ctype/isalnum_test.cpp
@@ -7,21 +7,17 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isalnum.h"
-#include "utils/UnitTest/Test.h"
 
-// Helper function that makes a call to isalnum a bit cleaner
-// for use with testing utilities, since it explicitly requires
-// a boolean value for EXPECT_TRUE and EXPECT_FALSE.
-bool call_isalnum(int c) { return __llvm_libc::isalnum(c); }
+#include "utils/UnitTest/Test.h"
 
 TEST(IsAlNum, DefaultLocale) {
   // Loops through all characters, verifying that numbers and letters
-  // return true and everything else returns false.
+  // return non-zero integer and everything else returns a zero.
   for (int c = 0; c < 255; ++c) {
     if (('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') ||
         ('0' <= c && c <= '9'))
-      EXPECT_TRUE(call_isalnum(c));
+      EXPECT_NE(__llvm_libc::isalnum(c), 0);
     else
-      EXPECT_FALSE(call_isalnum(c));
+      EXPECT_EQ(__llvm_libc::isalnum(c), 0);
   }
 }
diff --git a/libc/test/src/ctype/isalpha_test.cpp b/libc/test/src/ctype/isalpha_test.cpp
--- a/libc/test/src/ctype/isalpha_test.cpp
+++ b/libc/test/src/ctype/isalpha_test.cpp
@@ -7,20 +7,16 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isalpha.h"
-#include "utils/UnitTest/Test.h"
 
-// Helper function that makes a call to isalpha a bit cleaner
-// for use with testing utilities, since it explicitly requires
-// a boolean value for EXPECT_TRUE and EXPECT_FALSE.
-bool call_isalpha(int c) { return __llvm_libc::isalpha(c); }
+#include "utils/UnitTest/Test.h"
 
 TEST(IsAlpha, DefaultLocale) {
-  // Loops through all characters, verifying that letters return true
-  // and everything else returns false.
+  // Loops through all characters, verifying that letters return a
+  // non-zero integer and everything else returns zero.
   for (int ch = 0; ch < 255; ++ch) {
     if (('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z'))
-      EXPECT_TRUE(call_isalpha(ch));
+      EXPECT_NE(__llvm_libc::isalpha(ch), 0);
     else
-      EXPECT_FALSE(call_isalpha(ch));
+      EXPECT_EQ(__llvm_libc::isalpha(ch), 0);
   }
 }
diff --git a/libc/test/src/ctype/isdigit_test.cpp b/libc/test/src/ctype/isdigit_test.cpp
--- a/libc/test/src/ctype/isdigit_test.cpp
+++ b/libc/test/src/ctype/isdigit_test.cpp
@@ -7,20 +7,16 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/ctype/isdigit.h"
-#include "utils/UnitTest/Test.h"
 
-// Helper function that makes a call to isdigit a bit cleaner
-// for use with testing utilities, since it explicitly requires
-// a boolean value for EXPECT_TRUE and EXPECT_FALSE.
-bool call_isdigit(int c) { return __llvm_libc::isdigit(c); }
+#include "utils/UnitTest/Test.h"
 
 TEST(IsDigit, DefaultLocale) {
-  // Loops through all characters, verifying that numbers return true
-  // and everything else returns false.
+  // Loops through all characters, verifying that numbers return a
+  // non-zero integer and everything else returns zero.
   for (int ch = 0; ch < 255; ++ch) {
     if ('0' <= ch && ch <= '9')
-      EXPECT_TRUE(call_isdigit(ch));
+      EXPECT_NE(__llvm_libc::isdigit(ch), 0);
     else
-      EXPECT_FALSE(call_isdigit(ch));
+      EXPECT_EQ(__llvm_libc::isdigit(ch), 0);
   }
 }
diff --git a/libc/test/src/ctype/islower_test.cpp b/libc/test/src/ctype/islower_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/ctype/islower_test.cpp
@@ -0,0 +1,21 @@
+//===-- Unittests for islower----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/ctype/islower.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(IsLower, DefaultLocale) {
+  // Loops through all characters, verifying that lowercase letters
+  // return a non-zero integer and everything else returns zero.
+  for (int ch = 0; ch < 255; ++ch) {
+    if ('a' <= ch && ch <= 'z')
+      EXPECT_NE(__llvm_libc::islower(ch), 0);
+    else
+      EXPECT_EQ(__llvm_libc::islower(ch), 0);
+  }
+}
diff --git a/libc/test/src/ctype/isupper_test.cpp b/libc/test/src/ctype/isupper_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/ctype/isupper_test.cpp
@@ -0,0 +1,21 @@
+//===-- Unittests for isupper----------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/ctype/isupper.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(IsUpper, DefaultLocale) {
+  // Loops through all characters, verifying that uppercase letters
+  // return a non-zero integer and everything else returns zero.
+  for (int ch = 0; ch < 255; ++ch) {
+    if ('A' <= ch && ch <= 'Z')
+      EXPECT_NE(__llvm_libc::isupper(ch), 0);
+    else
+      EXPECT_EQ(__llvm_libc::isupper(ch), 0);
+  }
+}