In criminal law, the institution of legal capacity is central to determining the liability of the accused. Law and psychiatry amongst many other medical disciplines, interact at the justice system's surface to determine criminal liability in furtherance of the legal proceedings' enhancement. This paper aims to define the legal framework, relevancy and admissibility of psychiatric expert opinion when commissioned by the court. The predominant focus is on criminal, rather than civil proceedings in which the psychiatrist gives an expert opinion. The author aims to elaborate on the relevancy of psychiatric expert opinion in determining criminal liability in Indian law and the recent developments that have taken place in western countries in this regard. The paper emphasises the need to curb the gap between legal and medical insanity while interpreting and analysing the accused's guilt. Analysis of international law, criminal procedure legislation, and relevant case laws support the author's theory that an interdisciplinary approach is essential. Following this method, conclusions are drawn, and further recommendations are provided.