This Monograph discussed some defences to criminal liability, such as, insanity, insane delusion, irresistible impulse, diminished responsibility and automatism with substantial reference to homicide cases under the common law and some African jurisdictions. Though most of these defences derive their source from the M'Naghten Rules which now seems obsolete having brought the legal and medical professions into conflict, thereby calling for reforms some are caused by some factors outside the rules.