Loads a namespace if not already loaded. It takes an argument(symbol) which should be a quoted (require 'clojure.string) ;; => nil After requiring a namespace, we need to refer the functions in a fully qualified names. Like, (require 'clojure.string) ;; => nil (clojure.string/split "This is an example of require" #" " ) ;; => ["This" "is" "an" "example" "of" "require"] Other way is to alias a namespace at the time of requring using as.