The present review concentrates on the biological aspects of porcine T lymphocytes. Their ontogeny, subpopulations, localization and trafficking, and responses to pathogens are reviewed. The development of porcine T cells begins in the liver during the first trimester of fetal life and continues in the thymus from the second trimester until after birth. Porcine T cells are divided into two lineages, based on their possession of the [@@@]\rmalpha [@@@]β or γδ T-cell receptor. Porcine [@@@]\rmalpha [@@@]β T cells recognize antigens in a major histocompatibility complex (MHC)-restricted manner, whereas the γδ T cells recognize antigens in a MHC non-restricted fashion. The CD4+CD8− and CD4+CD8lo T cell subsets of [@@@]\rmalpha [@@@]β T cells recognize antigens presented in MHC class II molecules, while the CD4−CD8+ T cell subset recognizes antigens presented in MHC class I molecules. Porcine [@@@]\rmalpha [@@@]β T cells localize mainly in lymphoid tissues, whereas γδ T cells predominate in the blood and intestinal epithelium of pigs. Porcine CD8+ [@@@]\rmalpha [@@@]β T cells are a prominent T-cell subset during antiviral responses, while porcine CD4+ [@@@]\rmalpha [@@@]β T cell responses predominantly occur in bacterial and parasitic infections. Porcine γδ T cell responses have been reported in only a few infections. Porcine T cell responses are suppressed by some viruses and bacteria. The mechanisms of T cell suppression are not entirely known but reportedly include the killing of T cells, the inhibition of T cell activation and proliferation, the inhibition of antiviral cytokine production, and the induction of immunosuppressive cytokines.