The lymphocyte-derived helper T (Th) cells are critical regulators of the adaptive immune response and are associated with inflammatory disease. The most recently recognized Th-cell lineage, Th17, plays an important role in host defense against extracellular pathogens by secreting the proinflammatory cytokine, interleukin 17, and recruiting reactive oxygen species (ROS)-producing monocytes to the site of infection. However, accumulating evidence has implicated Th17-cell dysregulation as an underlying cause for some immune-related pathogenic conditions, including allograft rejection. Recent studies of human transplant patients have indicated that Th17 cells exhibit resistance to current immunosuppressive therapies that would otherwise prevent allograft rejection. In this review, we will discuss the most current research findings related to Th17-cell function in various kinds of allografts.