This dissertation deals with the comparative analysis of mental illness as a defence in criminal law. The mental illness / insanity defence is deemed applicable when the accused does not have mens rea or lacks criminal responsibility or is afflicted by the inability to appreciate the wrongfulness of his act and act accordingly, at the time of the commission of the offence due to a pathological disturbance of the mental faculties. A review of the law in South Africa, English Law and United States of America law was done with regard to their approach in connection with the matter. The legal systems of South Africa, English Law and the United States of America were compared and analyzed because English Law and United States of America are developed countries and I decided to compare their approach to insanity defence with reference to South Africa, which is a developing country. Similarities were drawn between South Africa and English Law and this could be attributed to the fact that South African law emanated from English law. This is an important research topic on comparative analysis of mental illness as a defence in criminal law. The law applicable today in South Africa in respect of the defence of mental illness is combined in the provisions of the Criminal Procedure Act 51 of 1977, which replaced the criteria as set out in the M’Naghten rules and the irresistible impulse test. In all the three countries law that were compared the burden of proof has always been on the accused to prove his case on a balance of probabilities but in South Africa the position now is he who alleges must prove because of the legislative amendments. United States of America law allows for the forcible medication with drugs of the mentally ill defendants who are charged with crimes so that they can be fit to stand trial. This is the only country in the ones that were analyzed, which practices such a barbaric and inhuman acts. In the USA , the defendant has the burden of proving the defence of insanity by clear and convincing evidence, and the finding in not guilty by reason of insanity, English law, South African law has the same finding in insanity cases. The most common diagnosis used in support of a defence of insanity continues to be schizophrenia in South Africa and in English law system. In the English law system, the Home Secretary has the power to order defendant to be detained in a hospital on the basis of reports from at least two medical practitioners that the defendant is suffering from mental illness, if the minister is of the opinion that it is in the public interest to do so. In South Africa, the accused will be detained in a psychiatric hospital or a prison pending the decision of a judge in chambers. The detention of those found not guilty by reason of insanity could be challenged under the Human Rights Act in English law because the legal definition of insanity is far wider than the medical concept of mental disorder. The Drs under English Law have to use the legal, not the medical understanding of the mental disorder. The placing of a burden of proof on the defendant may be challengeable under European Convention of Human Rights as contrary to the presumption of innocence that is protected under convention. Finally this is a controversial subject on mental illness but the position in South Africa has been clear for a long time, and I did not come across any deficiencies in our law. I submit that South African law position on mental illness is good.