This review article describes the different aetiological agents known or suspected to cause colitis in the adult horse, namelySalmonellaspp.,Clostridiumspp.,Ehrlichia risticii,Cyathostomes, fungi, various antibiotics, drugs, and toxins, with emphasis on their mechanism of action. For each of the infectious agents, diagnostic procedures are indicated. The effects of endotoxin can be important in all forms of equine colitis.