Although mast cells have long been considered the integral effector cell in allergy and atopic disease, the paradigm of mast cell function is now evolving to incorporate data showing that mast cells make innumerable contributions to both protective and pathologic immune responses. Mast cells express cell surface molecules with costimulatory or co-inhibitory activity and produce a multitude of mediators that can direct dendritic cell (DC) or T-cell differentiation and function. In addition, mast cells exhibit a widespread distribution and are in close proximity to DCs and T cells at several critical sites. While there has been amazing progress in characterizing mast cell populations in vitro, only recently has the ability to monitor their in vivo effects become a reality. In this review, we discuss the evolution of our understanding of mast cell biology with an emphasis on their established and hypothesized roles in influencing T-cell differentiation and function. The fact that T-cell and mast cell interactions exist and are a normal component of most adaptive immune responses is one of the best illustrations of the now established concept that innate and adaptive immunity are not completely independent entities.