This report delves into the critical process of diagnosing mental illness within healthcare, focusing on psychological approaches to patient care, specifically for those suffering from depression and schizophrenia. It provides a detailed analysis of both psychological and biological explanations for mental disorders, comparing and contrasting their causes and impacts. The report further explores diagnostic methods used in the UK, including the application of the DSM-IV-TR, examining its reliability, validity, and addressing cultural considerations. It also evaluates the effectiveness of current diagnostic systems, supported by a case study, to provide a comprehensive understanding of the complexities involved in diagnosing and treating mental illness. The report highlights the significance of patient-centric decision-making and the importance of accurate diagnosis for effective treatment plans.