The thiopurines azathioprine and 6-mercaptopurine are effective both for active disease and for maintaining remission in both Crohn's disease and ulcerative colitis. This review describes criteria for starting thiopurines (two or more courses of steroids in a calendar year, relapse as prednisolone is reduced below 15 mg/day, within 6 weeks of stopping steroids) and the benefits of continuing treatment for up to 5 years. Challenging issues, such as thiopurine intolerance, relative merits of azathioprine, 6-mercaptopurine, monitoring therapy and thiopurines in pregnancy are addressed.