PROJECT SUMMARY/ABSTRACT Cognitive disengagement syndrome (CDS; previously termed sluggish cognitive tempo) is a set of behavioral symptoms characterized by excessive daydreaming, slowed thinking, and mental confusion. Despite being overlooked in psychopathology research for decades, it is now established that CDS symptoms (1) are distinct from other psychopathology including ADHD and internalizing symptoms, (2) can be reliably measured, and (3) increase across development. CDS is also associated with several significant areas of functional impairment including internalizing symptoms, suicide risk, and interpersonal difficulties. As CDS research advances, there is a need for developmentally-informed research that can advance theoretical models of CDS within broader models of psychopathology. We propose that CDS may be an important yet understudied transdiagnostic vulnerability to psychopathology that can inform models of heterotypic comorbidity while also being an untested gateway to the development and rise of internalizing problems across adolescence. However, there is a dearth of research examining CDS during adolescence, particularly with a longitudinal design. To address this gap in the existing scientific evidence base, we recently recruited a large, diverse community sample (N=341; ages 10-12 years) enriched for CDS symptoms to ensure the full range of CDS was represented. Participants are assessed at three timepoints over a 2-year period (i.e., baseline, 1-year follow-up, 2-year follow-up). Retention rates currently exceed 93%. Given its size and scope, this study comprises the most rigorous CDS study to date. However, despite the ongoing study being the only CDS-specific longitudinal sample in adolescence, it is limited to 3 timepoints over a 2-year period when the maximum age of participants will be 14 years. We thus have a unique opportunity to leverage this large, highly unique sample by conducting 4 additional assessments which will result in a total of