Background: The dramatic impact of IO on treatment outcomes has heightened interest in predictive biomarkers, including genomic markers such as tumor mutational burden (TMB) and microsatellite instability (MSI). The recent FDA approval of pembrolizumab for previously treated advanced solid tumors with elevated TMB (≥10 mut/Mb on FoundationOne CDx, F1CDx) now requires a better understanding of the prevalence of this and other IO biomarkers detected on CGP, including differences between TMB detected in tissue and mutational burden detected in blood (bTMB). Methods: Tissue and plasma biopsies were profiled with two CGP panels of 324 genes with 0.8 Mb genome coverage (F1CDx and FoundationOne LiquidCDx). Mutational burden was calculated by counting somatic variants (single nucleotide and indels, including synonymous variants, excluding germline and driver mutations) with variant allele frequency (VAF) ≥5% in tissue (TMB) or ≥0.5% in ctDNA (bTMB). MSI score was assessed using 95 repetitive loci and principal component analysis (tissue) or >1,800 repetitive loci (plasma). ctDNA levels were estimated using composite tumor fraction (cTF), a metric based on aneuploidy and VAF. Results: Pan-cancer, TMB ≥10 was detected in 19% of tissue cases (29,238/156,294) and was common in melanoma (53%), small cell (41%), NSCLC (40%), bladder (39%), and endometrial (24%). bTMB ≥10 was detected in 13% of liquid biopsies (806/6,295); prevalence by cancer type was correlated with prevalence of elevated TMB (r = 0.81). Samples with bTMB ≥10 had an elevated cTF (median 13%, IQR 5 - 31%) as compared to samples with bTMB <10 (median 1.8%, IQR 0.6 - 7%, p < 0.001). Among 353 cases with both tissue and liquid CGP results (median 11 months apart), the relative prevalence of TMB ≥10 (12%) and bTMB ≥10 (13%) were similar, with concordant detection in 303 cases (86%). MSI-high (MSI-H) was seen in 2.2% of tissue CGP (3,461/156,294), most often in endometrial (19%), stomach (6.0%), and colorectal (5.3%) cancers, while MSI-H was detected in 0.68% of ctDNA specimens (43/6,295), which were also those with elevated cTF (median 11%, IQR 7 - 23%). Of 3,504 cases with MSI-H signature on tissue or liquid CGP, 1,619 (46%) had a pathogenic mutation detected in MLH1/MSH2/MSH6/PMS2 (15% predicted germline). CD274 amplification was detected in 1,207 cases (0.77%) of tissue CGP and 11 cases (0.17%) in ctDNA. Conclusions: Elevated bTMB is overall less prevalent than elevated tissue TMB, though these biomarkers are detected in similar cancer types. Detection of bTMB ≥10 and MSI-H in liquid biopsy was associated with elevated ctDNA levels, suggesting a limit of detection, and potentially indicating a more aggressive biology in samples positive for these biomarkers. Further investigation is needed to understand the utility of bTMB for identifying high TMB tumors that may benefit from IO.