Gastrointestinal stromal tumours (GIST) are the most common mesenchymal tumours of the alimentary tract. They normally involve the stomach, the small bowel, or the colon. Localisation within the oesophagus, rectum, mesentery, omentum, or retroperitoneum is less common. GISTs are immunohistochemically identified by the expression of the c-kit protein, which is not detected in other mesenchymal tumours. The role of imaging includes the detection (subjects with occult gastrointestinal bleeding, incidental recognition, etc.), characterisation, analysis of relations between mass and gastrointestinal wall, staging, prognostic assessment (recognition of signs of malignancy and unfavourable prognosis), and follow-up during specific treatment. Owing to the frequent exophytic growth of these lesions, differentiation of these tumours from nondigestive lesions of different nature is a common diagnostic problem. Imaging findings usually allow differentiation from gastrointestinal epithelial tumours but not from non-epithelial tumours, for which histological confirmation is necessary, in part to verify potential response to therapy. Smaller lesions, which are usually benign, tend to be well-defined, relatively homogeneous, and with intraluminal growth. Larger lesions normally show well-defined or ill-defined margins, inhomogeneous density both on unenhanced and on contrast-enhanced scans, with combined intraluminal/extraluminal growth and a tendency to spread to surrounding structures. Internal attenuation is often necrotic or clearly fluid. Signs of high-grade GIST include liver metastasis, gastrointestinal wall infiltration, large volume, irregular surface, ill-defined margins, inhomogeneous enhancement and peritoneal spread. Recurrences usually share the appearance of the larger, primary malignant GIST.