Gastrointestinal stromal tumors (GISTs) may be defined as morphologically spindle cell, epithelioid, or occasionally pleomorphic mesenchymal tumours of the gastrointestinal tract that usually express the KIT protein and harbour mutation of a gene that encodes for a type III receptor tyrosine kinase (either KIT or PDGFRA). In Caucasian populations their annual incidence is 10 to 15 cases per million. Approximately 80% of GISTs have mutated KIT and 5% mutated PDGFRA. Most KIT mutations occur in untreated GISTs in the juxtamembrane exon 11 and only rarely in the kinase domain, whereas in imatinib-treated patients secondary mutations are frequent in exons encoding for the ATP/imatinib binding pocket or the kinase activation loop. Surgery is the standard treatment of local GIST. Tyrosine kinase inhibitor imatinib is the standard treatment for metastatic disease with few exceptions. A majority (80–90%) of patients with metastatic disease respond to imatinib or achieve durable tumour growth stabilisation with continuous therapy using a daily dose of 400 mg to 600 mg. Treatment with imatinib increases survival of patients with advanced disease with a few years and is associated with only moderate toxicity. Imatinib is being evaluated as adjuvant treatment following surgery, and other tyrosine kinase inhibitors as treatments of advanced GIST.