The Tec family of tyrosine kinases consists of five members (Itk, Rlk, Tec, Btk, and Bmx) that are expressed predominantly in hematopoietic cells. The exceptions, Tee and Bmx, are also found in endothelial cells. Tee kinases constitute the second largest family of cytoplasmic protein. tyrosine kinases. While B cells express Btk and Tee, and T cells express Itk, Rlk, and Tee, all four of these kinases (Btk, Itk, Rlk, and Tee) can be detected in mast cells. This chapter will focus on the biochemical and cell biological data that have been accumulated regarding Itk, Rlk, Btk, and Tee. In particular, distinctions between the different Tee kinase family members will be highlighted, with a goal of providing insight into the unique functions of each kinase. The known functions of Tec kinases in T cell and mast cell signaling will then be described, with a particular focus on T cell receptor and mast cell Fc epsilon RI signaling pathways.