This review considers factors affecting the coexistence of closely related pathogen species on arable crops, with particular reference to data available at Rothamsted for Septoria tritici/Stagonospora nodorum (Mycosphaerella graminicola/Phaeosphaeria nodorum) (septoria leaf blotch diseases on winter wheat), Oculimacula yallundae/O. acuformis (eyespot disease of winter cereals), and Leptosphaeria maculans/L. biglobosa (phoma stem canker on winter oilseed rape). Factors affecting the short-term, medium-term, and long-term coexistence of such related pathogen species are reviewed, and their evolution from common ancestors considered. Small niche differences between the related pathogen species enable them to coexist on the same host. The niche differences result from small differences in their biology/epidemiology, leading to separation in space, time, or resource use. Changes in both natural (e.g., fluctuating temperature) and man-made (e.g., agronomic practices, pollution) factors influence the coexistence. Such factors may result in coexistence between the related species in some parts of the world, whereas in other parts only one species occurs. These principles illustrated with pathogens of arable crops are generic to other host-pathogen systems.