The Traf-linked tumor necrosis factor receptor family member CD27 is known as a T cell costimulatory molecule. We generated CD27-/- mice and found that CD27 makes essential contributions to mature CD4+ and CD8+ T cell function: CD27 supported antigen-specific expansion (but not effector cell maturation) of naïve T cells, independent of the cell cycle-promoting activities of CD28 and interleukin 2. Primary CD4+ and CD8+ T cell responses to influenza virus were impaired in CD27-/- mice. Effects of deleting the gene encoding CD27 were most profound on T cell memory, reflected by delayed response kinetics and reduction of CD8+ virus-specific T cell numbers to the level seen in the primary response. This demonstrates the requirement for a costimulatory receptor in the generation of T cell memory.