First discovered in the 1980's, granulysin has until recently been thought of solely as an effector molecule present within cytotoxic immune cell populations, and involved in the direct killing of pathogens and infected or transformed eukaryotic cells. However, recent research has suggested an additional role of granulysin, in particular the 15 kDa isoform. While 9 kDa granulysin is broadly cytotoxic and capable of the direct killing of bacteria and other pathogens, the 15 kDa isoform of this molecule has been shown to function as an immune 'alarmin', causing the maturation and migration of antigen-presenting cells and other cells of the immune system. This dual function of granulysin indirectly increases the immune response to an infection or tumour, and therefore escalates its importance in the immune system. Here we review the different roles of granulysin, both as a cytotoxic molecule, and as a modulator of the immune system, and discuss the impact this molecule may have on the response to tumour and infection.