Andy Piper <andy(a)xemacs.org&gt; writes: #'(lambda ...) is the reader shorthand for (function (lambda ...)), while '(lambda () is shorthand for (quote (lambda ...)). In interpreted mode, they're equivalent. When compiled, the latter instructs the byte-compiler to compile the lambda form. So (mapcar '(lambda ...) somelist) is slower[1] than (mapcar #'(lambda ...) somelist) To make the story just a bit more spicy, there's also the lambda macro that is a shorthand for (function (lambda ...)). Thus my favorite form of writing the above is: (mapcar (lambda ...) somelist) [1] Actually, that's a lie. In the special case of map* functions, when the byte-compiler finds that the argument is a constant list whose car is the symbol `lambda', it compiles the list. (defun byte-compile-funarg (form) ;; (mapcar '(lambda (x) ..) ..) ==> (mapcar (function (lambda (x) ..)) ..) ;; for cases where it's guaranteed that first arg will be used as a lambda. (byte-compile-normal-call (let ((fn (nth 1 form))) (if (and (eq (car-safe fn) 'quote) (eq (car-safe (nth 1 fn)) 'lambda)) (cons (car form) (cons (cons 'function (cdr fn)) (cdr (cdr form)))) form))))