Background: Despite the effectiveness of the various targeted therapies currently approved in solid tumors, acquired resistance remains a persistent problem that limits the ultimate effectiveness of these treatments. Polyclonal resistance to targeted therapy has been described in multiple solid tumors through high throughput analysis of multiple tumor tissue samples from a single patient. However, biopsies at the time of acquired resistance to targeted agents may not always be feasible and may not capture the genetic heterogeneity that could exist within a patient. We used here sequencing of circulating tumor DNA (ctDNA) to characterize the landscape of secondary resistance mechanisms in a large cohort of patients with solid tumors. Methods: This study enrolled patients with advanced cancer from two institutional molecular profiling program STING (NCT04932525, sponsor: Gustave Roussy) or BIP (NCT02534649sponsor: Institut Bergonié). Genomic analysis was performed for each patient by using the Foundation One Liquid CDx Assay (324 genes, tumor mutational burden [TMB], microsatellite instability status). Results: 3435 patients with metastatic disease entered the study. Among them 992 patients (29%) received a targeted therapy matched to a specific molecular alteration before ctDNA. The main tumor types were: prostate cancer (349, 35%), luminal breast cancer (236, 24%), oncogene-addicted non-small cell lung cancer (129, 13%), KRAS-wild type colorectal cancer (126, 13 %). The most frequent class of targeted agents were androgen receptor pathway inhibitor (n = 350, 35%), aromatase inhibitor (236, 24%), anti-EGFR monoclonal antibodies (166, 17%), anti-EGFR tyrosine kinase inhibitors (83, 8%). ctDNA sequencing revealed DNA aberrations involved in secondary resistance in 308 patients (31%). The most frequent aberrations were AR mutations/amplifications, ESR1 point mutations, KRAS point mutations, EGFR point mutations. Among patients with resistance mutation, polyclonal aberrations were identified in 123 patients (40%). The median number of polyclonal aberrations per patient was 2 (range: 2-16). Polyclonal aberrations involved at least 2 different genes in 32 patients (10%). Preliminary results suggest that patients with polyclonal aberrations had worse outcome in comparison with patients with one or no detected aberration and final data will be presented at the time of the congress. Conclusions: We report here the first comprehensive landscape of genomic aberrations in ctDNA involved in resistance to targeted therapies in cancer patients. Polyclonal secondary genomic aberrations represent a frequent clinical resistance mechanism that may explain the poor rate of sustained complete remission observed with targeted therapies and must guide the development of future combinatorial strategies.