diff --git a/Makefile b/Makefile
index cc7dff9..b045d6d 100644
--- a/Makefile
+++ b/Makefile
@@ -21,6 +21,7 @@ LIBS += $(shell pkg-config --libs xcb-keysyms xcb-dpms xcb-image xcb-xinerama)
 endif
 LIBS += -lpam
 LIBS += -lev
+LIBS += -lX11
 
 FILES:=$(wildcard *.c)
 FILES:=$(FILES:.c=.o)
diff --git a/README b/README
index 213f5ef..99e0af5 100644
--- a/README
+++ b/README
@@ -26,6 +26,7 @@ Requirements
 - libcairo-dev
 - libxcb-xinerama
 - libev
+- libx11-dev
 
 Running i3lock
 -------------