Longitudinal research studies follow patients over prolonged periods of time and collect repeated data measurements. In radiation oncology patients, these studies provide the ability to obtain information on the effects of treatment and the evolution of a patient's condition over time. There is a wide range of applications for longitudinal studies in the field of radiation oncology, including treatment response and duration, disease progression, patient-important outcomes, long-term toxicities, and exploratory data collection for a disease and corresponding treatment. This chapter will provide an overview of longitudinal studies, advantages and disadvantages, steps to developing a successful study, and applications in the field of radiation oncology.