diff --git a/arduino-mk/Arduino.mk b/arduino-mk/Arduino.mk
index 5ab69c8..d50ff22 100644
--- a/arduino-mk/Arduino.mk
+++ b/arduino-mk/Arduino.mk
@@ -547,8 +547,19 @@ CAT     = cat
 ECHO    = echo
 
 # General arguments
-SYS_LIBS      = $(wildcard $(patsubst %,$(ARDUINO_LIB_PATH)/%,$(ARDUINO_LIBS)))
 USER_LIBS     = $(wildcard $(patsubst %,$(USER_LIB_PATH)/%,$(ARDUINO_LIBS)))
+USER_LIB_NAMES= $(patsubst $(USER_LIB_PATH)/%,%,$(USER_LIBS))
+
+# Let user libraries override system ones.
+SYS_LIBS      = $(wildcard $(patsubst %,$(ARDUINO_LIB_PATH)/%,$(filter-out $(USER_LIB_NAMES),$(ARDUINO_LIBS))))
+SYS_LIB_NAMES = $(patsubst $(ARDUINO_LIB_PATH)/%,%,$(SYS_LIBS))
+
+# Error here if any are missing.
+LIBS_NOT_FOUND = $(filter-out $(USER_LIB_NAMES) $(SYS_LIB_NAMES),$(ARDUINO_LIBS))
+ifneq (,$(strip $(LIBS_NOT_FOUND)))
+    $(error The following libraries specified in ARDUINO_LIBS could not be found (searched USER_LIB_PATH and ARDUINO_LIB_PATH): $(LIBS_NOT_FOUND))
+endif
+
 SYS_INCLUDES  = $(patsubst %,-I%,$(SYS_LIBS))
 USER_INCLUDES = $(patsubst %,-I%,$(USER_LIBS))
 LIB_C_SRCS    = $(wildcard $(patsubst %,%/*.c,$(SYS_LIBS)))