T helper (Th) type 17 cells are a recently described CD4+ T-cell subset that may contribute to allograft rejection and act as a barrier to the induction of transplant tolerance. This review examines the involvement of Th17 cells in transplant rejection, how immunosuppressive medication may affect their induction and maintenance and the potential plasticity of developing Th17 cells. It also addresses the complex interplay between the Th17 and regulatory T-cell developmental pathways and the susceptibility of Th17 cells to regulation. Despite accumulating evidence, the precise impact of Th17 cells on transplant rejection and the induction of tolerance require further clarification.