This chapter is devoted to the idea of ‘Mental Disorder’. It commences with a discussion of Buddhist ideas about dukkha or unsatisfactoriness, and the varying extents to which this concept relates to the fundamental nature of reality, human unhappiness, the ups and downs of everyday life, reactions to prevailing social orders, and states of mind which we now consider to be ‘mental health conditions’, ‘mental illnesses’, or ‘mental disorder’. This chapter also explores ‘mental disorder’ in international classification systems in psychiatry, including the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders (Fifth Edition) (Text Revision) (DSM-5-TR) and the World Health Organization’s ICD-11: International Classification of Diseases (Eleventh Revision) (ICD-11), as well as general approaches to diagnosis in clinical practice. The chapter concludes by examining the relevance of these concepts and various Buddhist ideas (e.g., dukkha, impermanence, dependent arising, non-self) to responses to suffering and the diagnosis of mental health conditions today. Throughout the chapter, there is an emphasis on different systems of interpreting and patterning human suffering, the commonalities across Buddhist and other traditions, and the ways in which these perspectives offer different opportunities for understanding.