T cells are critical for immunity to malaria, not only because they function as helper cells for an antibody response, but also because they serve as effector cells. Such cellular immunity is directly implicated in protection from sporozoites and plays an important role in protection from blood-stage parasites. It also can block transmission of malaria from mammalian host to the mosquito. Both CD8 and CD4 effector cells have important roles. The parasite's defence from immune attack, however, is designed to minimize activation of T cells. Thus, there appears to be limitation of the number of T sites within many malaria proteins and variation within these limited sites. Homology to host proteins and resultant immune-escape due to tolerance may be another mechanism. These parasite defence mechanisms highlight both the importance of T-cell immunity in malaria and the challenge of designing effective vaccines to stimulate T cells.