 Gastrointestinal stromal tumors GISTs belong to the sarcoma group and are characterized by oncogenic mutations in the c-KIT, PDGFRA, BRAF and NF-1 genes that drive tumor growth. Since tyrosine kinase inhibitors TKIs have become available, the median survival of GIST patients increased from 9 months to over 5 years. Consequently, this rare disease has become a role model for other targeted therapies. However, response to TKIs is extremely heterogeneous: 15 of the patients experience no benefit from imatinib, whereas 17 of the patients enjoy stable disease for over 9 years. Treatment failure due to primary and secondary resistance is caused in part by mutations in oncogenic genes that cause change in drug sensitivity. A new technique, using circulating tumor DNA, has enabled us to assess mutations in a simple blood sample obtained from patients on treatment, and thus detect new mutations early in the course of the disease. Also, differences in pharmacokinetic drug behavior add to the observed heterogeneity, and may cause resistance due to drug underexposure and thereby proliferation of the least sensitive tumor cells. This offers the opportunity to optimize and personalize targeted treatment for individual GIST patients by timely treatment adaptation based on early detection of secondary TKIs resistance mutations. Achieving this urgently requires data on daily clinical practice, including prospective serial mutation analysis and serial drug plasma concentration measurement. At a fundamental level this will also help to unravel the driving factors behind primary and secondary TKIs resistance in this model disease. 