Background: High-Grade Gliomas (HGGs) are the most aggressive primary brain tumors in adults and molecular characterization is crucial for diagnosis and optimal disease treatment. Due to the eloquent location of many HGGs, upfront or repeat tumor sampling may be sub-optimal. Hence, there is an urgent need to develop alternative, minimally invasive means to obtain diagnostic information and determine the expected clinical behavior. Here we report that circulating tumor DNA (ctDNA) from cerebrospinal fluid (CSF) can be used to identify disease defining genomic alterations and to track clonal evolution. Additionally, we demonstrate that detection of CSF ctDNA is positively correlated with leptomeningeal disease and overall survival suggesting that CSF ctDNA should be integrated into clinical decision making in the clinic. Methods: Our study includes 313 samples from 253 patients with recurrent glioma treated at Memorial Sloan Kettering Cancer Center who underwent CSF collection for routine clinical care and were sequenced using the MSK-IMPACT targeted clinical sequencing assay (468 and 505 genes). Alterations were classified as oncogenic based on OncoKB. 17% of genomic alterations detected were identified using a secondary bioinformatics analysis, following an informed approach with less stringent mutation calling criteria and Gaussian Mixture Model to call copy number events. CSF ctDNA positivity was determined by the presence of at least one oncogenic disease defining alteration or any shared alteration with the tumor. Results: Within this cohort, we found 193 CSF ctDNA positive (62%) and 120 CSF negative (38%) samples. Of note, CSF ctDNA positivity was the highest amongst histone mutant tumors with 94% CSF ctDNA positivity compared to 56% CSF ctDNA positivity for IDH-WT tumors. We noticed 44% of alterations shared between the tumor and the CSF, additionally, we noted considerable tumor evolution particularly within the growth signaling pathways (e.g. PDGFRA). The majority of the shared alterations were clonal, and we noticed the emergence of new clonal events in the CSF. Conclusions: Our cohort demonstrated that patients with positive CSF ctDNA had a significantly shorter overall survival compared to those who were CSF ctDNA negative (4.83 months vs 11.83 months, HR 2.1, p < 0.001). In tandem with secondary clinical analysis, radiographic findings also correlated with CSF ctDNA positivity. Specifically, the presence of enhancing disease and contact of the tumor with the ventricular space were positively associated with detection of CSF ctDNA. Additionally, CSF ctDNA was associated with positive cytology in 21/21 cases (100%). With our improved bioinformatics pipeline, we hypothesize ctDNA from CSF may be used as a prognostic biomarker for survival, but confirmation requires further validation in a prospective study.