Desmoid tumours (DT) are slow-growing tumours that consist of proliferations of well-differentiated fibroblasts. Although the typical characteristics of malignant tumours, such as distant metastases, are absent, the tumours are locally aggressive and grow into neighbouring structures. The prevalence of desmoid tumours in patients with FAP is 7-12%. The lifetime risk of developing desmoid tumours is about 20%. In FAP, most tumours are intra-abdominal or located in the abdominal wall. Next to colorectal cancer, desmoid tumours are the most frequent cause of death in FAP. Possible risk factors for the development of desmoid tumours are previous surgical procedures, pregnancy, female sex, a family history of desmoid tumours, and specific mutations in the APC-gene. Both CT scanning and MRI can be used to detect the tumours. An excision biopsy is needed to establish the diagnosis. Medicinal treatment with NSAIDs is the treatment of first choice, followed by hormonal treatment (e.g., tamoxifen) in combination with NSAIDs. Both forms of treatment lead to a response in about 30-50% of the patients. Surgery is the preferred treatment for extra-abdominal tumours or tumours located in the abdominal wall. Surgical treatment of intra-abdominal tumours is only indicated in patients with obstruction of the bowel or ureter. Chemotherapy is indicated in patients with progressive desmoid tumours when non-cytotoxic treatment has failed. Radiotherapy may play a role in the treatment of irresectable extra-abdominal or abdominal wall tumours, or as adjuvant treatment of tumours with positive margins.