Over the course of their Crohn's disease, up to half of affected individuals will develop a perianal penetrating complication (either perianal fistula or abscess). Symptoms can be debilitating, and the impact on quality of life significant. This first edition of 'Fast Facts: Complex Perianal Fistulas in Crohn's Disease' explains how fistulas form, who is at risk and the principles of diagnosis and classification. Later chapters explore the medical and surgical options, including innovative therapies. The intention throughout is to emphasize the importance of ‘joined-up thinking’ when caring for patients. Perianal fistulas are often challenging to treat in patients with Crohn’s disease, and a multidisciplinary approach is needed to get the best outcomes.