Cas-L (Crk-associated substrate lymphocyte type), also known as HEF1 (human enhancer of filamentation 1) or NEDD9 (neural precursor cell-expressed, developmentally downregulated gene 9), is an adapter protein at focal adhesions and transmits various signals, mainly induced by integrins. Cas-L is a member of the Cas family proteins and is expressed preferentially in lymphocytes and epithelial cells. A number of previous studies have suggested that Cas-L plays an important role in lymphocyte movement and cell cycle. Recently, we reported a novel function of Cas-L in the immune system using gene-targeted mice. The Cas-L-deficient lymphocytes showed insufficient chemotactic response and perturbed cell adhesion. Moreover, a deficit of marginal zone (MZ) B cells was detected in the mutant mice. Here, we review the structure of Cas-L and several signaling pathways in which Cas-L is involved, based on the previous in vitro studies. Subsequently, biological functions of Cas-L and the relevance of Cas-L to human diseases are discussed.