Parasitism was recognized as a potent evolutionary force by Haldane as early as 1949 but was then largely ignored by evolutionary biologists until the mid-1970s. Interest has since been rekindled, and parasites have now been attributed major roles in areas of evolutionary biology as fundamental and diverse as social behaviour, sexual reproduction, ornamentation and speciation. These ideas have their roots in evolutionary theory, and parasitologists have as yet contributed little to their development and testing. Likewise, evolutionary biologists have been slow to assimilate the parasitological literature. This year's BSP Autumn Symposium was thus organized with the aim of promoting constructive interchange between the two disciplines.