When the immune system encounters alloantigen it can respond in any one of a number of different ways. The choice that is made will take into account factors such as where, when and how the contact with the alloantigen takes place, as well as the environmental conditions that prevail at the time the alloantigen is encountered. Alloantigen administration before transplantation either alone or in combination with therapeutic agents that modulate the functional activity of the responding leucocytes can be a powerful way of inducing specific unresponsiveness to alloantigens in vivo. The molecular mechanisms that influence the way the outcome of the immune response to alloantigen develops, either activation or unresponsiveness to the triggering antigen, hold the key to our ability to manipulate the immune system effectively by exposing it to donor antigen for therapeutic purposes. This review will focus on alloantigen-induced immunological unresponsiveness and how insights into the mechanisms of unresponsiveness have driven the development of novel tolerance-induction strategies that show promise for translation into the clinic in the future.