This article presents a rare longitudinal analysis of solitary confinement use in one state prison system: spanning 2002–2017 in the Washington Department of Corrections (DOC). An ongoing partnership with DOC officials facilitated methodological and conceptual improvements, allowing us to construct a dataset that provides a rich description of who is in solitary confinement, for how long, and why. Operationalizing solitary confinement as the intersection of the most serious custody status with the most restrictive housing location, we describe significant changes in ethnic composition and behavioral profiles of people in solitary confinement and in frequency and duration of solitary confinement use. These results suggest how particular policy interventions have affected the composition, numbers, and lengths of stay in solitary confinement. Combining longitudinal analysis and iterative engagement with DOC officials, we provide a roadmap for better understanding solitary confinement use in the United States now and in the future.