The current study addresses the fundamental role of expert evidence advanced in support of the defence of criminal incapacity. It was endeavoured to illustrate that the scientific entities of forensic psychiatry and psychology fulfil an essential and pivotal role in establishing and assessing the defence of criminal incapacity. The study proposed to illustrate the interaction between the professions of law and medicine on the backdrop of the defence of criminal incapacity. Recommendations were provided with the aim of enhancing the dialogue between the professions of law and medicine when the defence of criminal incapacity falls to be assessed. The study was approached from a dual dimensional perspective illustrating both the need for mental health experts as well as the need for adequately trained and experienced mental health experts to provide expert testimony as to an accused’s mental state when the defence of criminal incapacity is raised. The motivation for the current study is enumerated and the concepts of “criminal capacity”, “non-pathological criminal incapacity”, “pathological criminal incapacity” and “expert evidence” are, amongst others, conceptualized. It is indicated that expert evidence plays an essential role not only in cases where pathological criminal incapacity, or put differently, criminal incapacity attributable to mental illness or mental defect is raised, but also in instances where non-pathological criminal incapacity is raised as a defence. The role of the mental health expert is addressed with reference to battered woman syndrome evidence advanced in support of the defence of non-pathological criminal incapacity. It is illustrated that the defence of non-pathological criminal incapacity is in need of reform. It is in addition illustrated that legislative reform is essential to establish the defence of non-pathological criminal incapacity and to create legal certainty. The inconsistent approach in the application of expert evidence to the defence of criminal incapacity is emphasized with specific focus on the semantic distinction between the defences of non-pathological criminal incapacity and pathological criminal incapacity. The role and application of the DSM-IV in the definition and assessment of mental disorders is addressed in conjunction with the various obstacles associated with the application of the DSM-IV to the defence of criminal incapacity. The nature and scope of the basic rules of expert evidence as they would apply to mental health professionals acting as expert witnesses in support of the defence of criminal incapacity are addressed. The assessment of the probative value of expert evidence is addressed and the complexities associated therewith are espoused. The numerous ethical dilemmas faced by mental health experts are illustrated and recommendations are provided aimed at eliminating these dilemmas. A comparative study of selected principles pertaining to expert evidence in the United States of America is embarked upon to illustrate the need for a codification of the rules of expert evidence as well as effective guidelines aimed at enhancing the scientific reliability and validity of expert evidence advanced in support of the defence of criminal incapacity. Finally, conclusions are drawn and motivated recommendations are made. Law reform is proposed in the form of draft proposals for legislative reform in respect of the defence of criminal incapacity as well as a draft ethical code of conduct for mental health experts providing expert testimony in cases where the defence of criminal incapacity is raised.