In health-related studies, researchers often collect data from the same unit (or subject) repeatedly over time. Measurements may be taken at different times for different subjects. These are called longitudinal studies. Diggle, Liang, and Zeger (1994) offer an excellent exposition of the issues related to the design of such studies and the analysis of longitudinal data. They also provide many interesting examples of data. We refer to their book for a thorough treatment of the topic. The purpose of this chapter is to introduce the methods based on recursive partitioning and to compare the analyses of longitudinal data using different approaches.