In recent years, there has been growing empirical interest in examining the role of affect dynamics in mental health. However, research on affect has largely progressed independently in the basic and applied sciences, yielding significant advances in each domain but little cross-disciplinary integration. This special issue addresses this gap by showcasing some of the most promising recent developments in the field. The articles featured in this special issue offer insights into key innovations in affect dynamics and their potential implications for mental health interventions. Comprising a total of 17 articles, the issue is divided into two sections: Daily Life Assessment of Affect, encompassing seven articles, and In-Treatment Assessment of Affect, comprising 10 articles. In this editorial, we synthesize the contributions of these articles and propose a set of fundamental principles for conducting and interpreting research on the role of affect dynamics as mechanisms of change in mental health interventions. These principles encompass (a) the content of affect research related to mental health and its treatment (the What), (b) the timing of the assessment (the When), (c) the target populations under investigation (the Who), and (d) the methodologies employed (the How). The synthesis presented here, along with the articles featured in this special issue, holds significant potential to inform clinical research and practice on the role of affect dynamics in mental health interventions and stimulate future scientific inquiry in this important area. (PsycInfo Database Record (c) 2024 APA, all rights reserved).