The meaning of McNaughten rules is assessed, with particular emphasis on the concepts of blameworthiness, dangerousness, and dissociation. Also discussed are the nature and quality of the act, knowledge of wrongdoing, irresistible impulse, the burden of proof, and the consequences of an insanity verdict. Arguments for and against abolishing the insanity defense also are provided, and the need for reform is discussed. Reforms relate to the definition of insanity, alternative tests, the costs and benefits of broad versus narrow tests, and the custodial consequences of an insanity finding (i.e., civil or criminal detention). Release decisionmaking also is discussed. Mental impairment is considered with reference to the problem of competing defenses -- automatism, mistake or misapprehension, amnesia, denial of criminal negligence, loss of self-control, and diminished responsibility. Law regarding fitness to stand trial also is considered. Arguments for and against the diminished responsibility defense, and definitional, evidentiary, and sentencing issues are discussed. Infanticide is examined with respect to mental disturbance and diminished responsibility. Finally, issues relating to intoxication and criminal responsibility and options for dealing with these issues are examined.