Homeostatic control of CD8 T cell populations is essential for defense against infectious pathogens. Our understanding of the mechanisms that control naı̈ve, effector and memory T cell populations in the intact animal has increased significantly over the last several years. There have been some surprises. For example, peripheral tissues have been found to harbor unexpectedly large numbers of effector memory T cells. Also unexpected was the finding of programmed T cell proliferation following very brief exposure to antigen. These and other recent advances are summarized in the following review.