{Reference Type}: Review {Title}: A classification of combined homicide-suicide: An update-Part I: Introduction and psychopathology. {Author}: Felthous AR;Masood Y;Kahlon C;Safari D;Rodgers E;Chennu N;Angly M; {Journal}: J Forensic Sci {Volume}: 69 {Issue}: 1 {Year}: 2024 Jan 2 {Factor}: 1.717 {DOI}: 10.1111/1556-4029.15391 {Abstract}: A 1995 classification of combined homicide-suicide was based on the psychopathology of the perpetrator and the relationship between the perpetrator and the victim(s). A three-part model was proposed for the study and potential understanding of individual homicide-suicide acts. This triarchic model consisted of the concepts of ego weakness, stress, and vector, the vector representing the ideational component. Today, approaching a quarter of a century after this initial classification based on a review of the literature available then, a considerable volume of experience and investigative knowledge has been published advancing our knowledge of homicide-suicide. The present review updates and revises the original classification, retains the two-part, psychopathological and relational classification, and adds several categories. Because this updated classification is more expansive than the original, it is presented in three parts. Part I, the present article, provides the introduction to this classification and the classification of mental conditions that may attend homicide-suicide. Parts II and III will address intrafamilial and extrafamilial homicide-suicide, respectively, the two major divisions of the classification based on the relationship between the actor and the homicide victim(s). All three parts are integral to this classification and belong together.