Background: The emergence of osimertinib (osi) as standard of care therapy for EGFR-mutant NSCLC has led to investigations into understanding and overcoming drug resistance. There are now a number of therapeutic approaches aimed at overcoming EGFR resistance mutations (muts). We sought to understand the biology of EGFR C797S and other EGFR resistance muts through querying our clinico-genomic database (CGDB). Methods: CGP results from tissue (n = 60,889) or circulating tumor DNA (ctDNA; [n = 9,922]) samples from 70,811 NSCLC patients (pts) were queried for known osi resistance muts in EGFR (C797, L792, G796, L718, G724). Clinical outcomes were evaluated for a cohort of NSCLC pts with osi resistance from the Flatiron Health-Foundation Medicine CGDB, a nationwide de-identified EHR-derived database linked to CGP data. Results: Between 12/2014 and 11/2020, 261 osi resistance mutations in EGFR were detected in 228 samples. The most common were C797S (66%), L718X (14%), G724S (11%), and others (9%). 173 C797S muts were detected in 155 samples (123 ex19del, 30 L858R, 2 other EGFR muts); 100 tissue, 55 ctDNA (median VAF = 7.6%). EGFR T790M co-occurred with C797S muts (96% cis, 3.7% trans) in 118 (76%) samples and decreased over time, occurring in 92% (24/26) of C797S samples tested in 2017 vs 56% (20/36) of samples tested in 2020 (p = 0.002). In 19/155 (12%) samples with C797S (14 ctDNA), multiple changes resulting in EGFR resistance muts were present: 16 samples had > 1 nucleotide changes resulting in C797S (100% trans), 3 samples had other resistance muts (L718Q/V, L792H, L792F) and 3 samples had multiple C797S changes with other resistance muts (C797G, L792H/F + G796S, L718Q + G796S+C797G). 29 pts (14 ctDNA) had C797S with potential off-target resistance (17 PIK3CA muts, 4 BRAF muts, 3 CCDC6-RET fusions, 3 KRAS muts, 2 ERBB2 amplifications (amps), 1 ERBB2 ex16 del, 1 STRN-ALK fusion, 1 FGFR3-TACC3 fusion). In the CGDB, 527 EGFR-mut NSCLC pts had documented receipt of osi. Pre and post osi-treated specimens were available for 19 of these pts (12 ex19del, 6 L858R, 1 G719A/S768I). Heterogeneous acquired resistance mechanisms were observed in the post-osi specimen, including 2 CCDC6-RET fusions, 2 MET amps, 2 BRAF fusions, BRAF V600E, and secondary EGFR muts (C797S, L704F, L718V). 161/527 pts had a documented line of therapy after osi discontinuation and most frequently received platinum doublet + immunotherapy (27%) or platinum doublet alone (23%); 17 (11%) pts received another EGFR tyrosine kinase inhibitor. 214/527 had documented osi progression and median post-progression survival was 11.8 months. Conclusions: Osi resistance in EGFR-mutant NSCLC is a poor prognosis condition. EGFR C797S is a recurring resistance mut which, in a minority of cases, can co-occur with alternate on and off target resistance muts detected with tissue and liquid biopsy.