diff --git a/httpcore/_backends/__init__.py b/httpcore/_backends/__init__.py
index e69de29b..e96118bb 100644
--- a/httpcore/_backends/__init__.py
+++ b/httpcore/_backends/__init__.py
@@ -0,0 +1,17 @@
+import os
+import sys
+import warnings
+
+# [Windows Only] SSLKEYLOGFILE can trigger the "no OPENSSL_Applink" error.
+# OpenSSL on Windows requires an 'Applink' interface to safely perform I/O
+# (like logging keys) across different C-Runtimes (CRT). Since CPython doesn't
+# provide this bridge and manages I/O independently, OpenSSL fails to access
+# Windows stdio, leading to a crash or the "no OPENSSL_Applink" error.
+if os.environ.get("SSLKEYLOGFILE") and sys.platform == "win32":
+    warnings.warn(
+        "Detected SSLKEYLOGFILE environment variable, which may cause "
+        "'no OPENSSL_Applink' errors. If you encounter this issue, try "
+        "unsetting or removing the variable temporarily.",
+        UserWarning,
+        2,
+    )