Gastrointestinal stromal tumors are soft tissue sarcomas of the gastrointestinal tract that originate from mesenchymal cells. Advances in the systemic therapy of gastrointestinal stromal tumors are highlighted by the rapid development and approval of the molecularly targeted therapy imatinib mesylate (Gleevec/Glivec). Mutations of the KIT gene are known to be present in most gastrointestinal stromal tumors and result in gain of function, with permanent activation of the expressed KIT receptor in the absence of binding of the stem cell factor ligand. Imatinib is the first rationally designed selective inhibitor of specific protein tyrosine kinases, including KIT. Inhibiting the downstream signaling of KIT switches the cell balance into apoptosis. Although complete responses have seldom been seen up until now, imatinib has proven to be extremely effective in the treatment of patients with unresectable and/or metastatic gastrointestinal stromal tumors.