Developing T cells are highly motile and undergo long-range migrations in the thymus as part of their developmental program. In the past two years, significant advances have been made in understanding the nature of the signals that control the entry of thymocyte progenitors into the thymus and the exit of mature thymocytes from the thymus. Progress has also been made in identifying the chemokine signals that control intrathymic migration patterns. In addition, the recent application of two-photon laser scanning microscopy has made it possible to make real-time observations of thymocytes within the three-dimensional environment of the thymus, and has shed new light on the relationship between positive selection and thymocyte migration.