Jiirgen Westermann and Reinhard Pabst harles Mackay first intro-duced the concept that ‘naive’T cells selectively migrate into lymphoid organs and ‘memory’T cells migrate into nonlymphoid organs’l. By revealing a connection between T-~ ell function and migration pathway2, this concept has substantially stimulated research on lymphocyte migration, and has influenced several other fieids in immunology. Moreover, the concept has become generalized. and many review articles have tended to report it as an established fact2-7, even though it is based mainly on indirect evidence obtained in one speck. This article considers the concept in the context of lymphocyte migrationk12, and discusses recent findings on the migration of naive and memory T cells i> r viva. In migration studies. it i5 necessary to identify naive and memory T cells in individual organs and their compartments through expression of cell-surface antigens. However, it is always problematic to deduce functions from phenotypic analysiP, and the definition of such T-cell subsets can: ary considerably between different species. Box 1 and Table 1 provide basic information regarding the mohxules used to define naive and memory T cells, and indicate the problems with allocating such definitions.