Regulatory T (T(reg)) cells expressing forkhead box P3 (Foxp3) arise during thymic selection among thymocytes with modestly self-reactive T cell receptors. In vitro studies suggest Foxp3 can also be induced among peripheral CD4(+) T cells in a cytokine dependent manner. T(reg) cells of thymic or peripheral origin may serve different functions in vivo, but both populations are phenotypically indistinguishable in wild-type mice. Here we show that mice with a Carma1 point mutation lack thymic CD4(+)Foxp3(+) T(reg) cells and demonstrate a cell-intrinsic requirement for CARMA1 in thymic Foxp3 induction. However, peripheral Carma1-deficient T(reg) cells could be generated and expanded in vitro in response to the cytokines transforming growth factor beta (TGFbeta) and interleukin-2 (IL-2). In vivo, a small peripheral T(reg) pool existed that was enriched at mucosal sites and could expand systemically after infection with mouse cytomegalovirus (MCMV). Our data provide genetic evidence for two distinct mechanisms controlling regulatory T cell lineage commitment. Furthermore, we show that peripheral T(reg) cells are a dynamic population that may expand to limit immunopathology or promote chronic infection.