Background: GIST is the most common mesenchymal cancer of the digestive tract. Beyond surgery, treatment for GIST focuses largely on tyrosine kinase inhibitors (TKI), whose selection and potential resistance depend on select mutations. We present the molecular landscape of GIST utilizing tissue and liquid biopsies with emphasis on the clinical utility of liquid biopsy in advanced GIST. Methods: Liquid (FoundationOne Liquid CDx [F1LCDx]) and tissue (FoundationOne CDx) CGP was performed by hybrid capture, targeted NGS at Foundation Medicine Inc. Tissue and liquid samples from 2,198 and 147 patients, respectively, were analyzed. A cohort of 27 paired tissue and liquid samples were also evaluated. The levels of circulating tumor DNA (ctDNA) in liquid biopsies was quantified by tumor fraction (TF), with a TF algorithm incorporating aneuploidy, variant allele frequency, and canonical alterations detected on F1LCDx. Results: Tissue CGP (n = 2,198) revealed the following prevalence of primary driver alterations: KIT (77%), PDGFRA (8%), NF1 (6%), SDHA/B/C/D (SDHx, 3%) and BRAF (1%). Rates of molecular markers previously associated with worse prognosis included: CDKN2A (29%), RB1 (9%), TP53 (6%) and SETD2 (4%). 7% of cases had no reportable known pathogenic alterations in canonical GIST genes (wild-type GIST), while 2% of cases had a mutation in more than one driver. In a cohort of 147 liquid biopsies, TF was < 1% in 68.0%, 1-10% in 18.4%, > 10% in 13.6% of samples. In samples with elevated TF ( > 10%), the prevalence of targetable driver alterations in KIT (89%), PDGFRA (4%), NF1 (4%) and BRAF (4%) was comparable to the tissue prevalence. In liquid, 58% (39/67) of samples with a KIT-driver mutation had a co-occurring imatinib-resistant KIT alteration. In addition, 4/147 patients (3%) were predicted to harbor a germline KIT mutation, including one patient (0.6%) with a potential imatinib-resistant KIT D820G germline mutation and another with clinical suspicion of germline KIT L576P mutation due to the presence of multiple primary GISTs, hyperplasia of myenteric plexus and dysplastic skin nevi. In paired tissue/liquid samples, liquid detected 2/2 driver mutations found in tissue when liquid TF was > 10%, and 5/6 in specimens with TF > 1%. In the overall cohort, the relative prevalence of KIT exon 9 and 11 driver alterations was comparable in tissue vs liquid, while imatinib-resistance KIT exon 13 and 17 mutations were enriched in liquid samples. Conclusions: Known driver and TKI-resistant mutations of both somatic and potential germline origin are identified in peripheral blood ctDNA of GIST patients. Liquid biopsy shows high concordance to tissue in identifying driver mutations in the presence of elevated TF and may exhibit TKI-resistant specific alterations. This study indicates that liquid biopsy may be useful in the molecular classification of GIST during the medical management of advanced GIST patients.