diff --git a/jmespath/functions.py b/jmespath/functions.py
index 31dab051..bd486c58 100644
--- a/jmespath/functions.py
+++ b/jmespath/functions.py
@@ -164,6 +164,14 @@ def _subtype_check(self, current, allowed_subtypes, types, function_name):
     @signature({'types': ['number']})
     def _func_abs(self, arg):
         return abs(arg)
+    
+    @signature({'types': ['string']})
+    def _func_lower(self, arg):
+        return arg.lower()
+    
+    @signature({'types': ['string']})
+    def _func_upper(self, arg):
+        return arg.upper()
 
     @signature({'types': ['array-number']})
     def _func_avg(self, arg):