Background: Next-generation sequencing (NGS) of plasma ctDNA is a promising non-invasive method to detect somatic genomic alterations (GAs). We investigated whether serial ctDNA measurements can predict disease progression and map the molecular evolution of advanced UC (aUC) through successive treatments. Methods: We analyzed cohorts from 2 academic centers, WCM and UNLV, with serial (≥2) ctDNA targeted NGS using the Guardant360 panel of 73 genes, ctDNA was collected within four weeks of restaging scans. Non-progressive disease (non-PD) was defined as stable disease or radiological response. Results: Our cohort included 176 individual ctDNA samples (median 3/patient) from 52 patients (38 men), the median age was 69 (range 38-88), 38 pts (73%) had metastatic disease, 20 pts (38.5%) had visceral metastases and 35 pts (67%) received first-line platinum-based combination chemotherapy for aUC. The median number of lines of therapy was 2. 49 pts (94%) had detectable GAs in at least one ctDNA sample. Most commonly identified GAs involved TP53 (67%), PIK3CA, EGFR, and ERBB2 (each in 19%). FGFR3 GAs were identified in 9 pts (17%, 2 FGFR3-TACC3 fusions, and 7 SNVs, including two novel mutations P250R and K650M). The mean variant allele fraction (VAF) significantly decreased in pts with non-PD (80 non-PD events) compared to pts with PD (45 PD events) with a mean VAF decrease of 2.06 % (p< 0.0001). This effect was consistent independent of the number of prior lines of therapy. The mean number of GAs per patient at each time point was significantly decreased in non-PD vs. PD group (2.8 vs. 4.4, p=0.004). Pts with detectable VAF in somatic alterations at any time point during the disease course had shorter overall survival (HR 1.18, 95% CI: 1.08 – 1.28, p<0.0001). Pts with sustained partial or complete responses were more likely to have VAF ≤0.2% suggesting that molecular remission was associated with clinical remission. Conclusions: Serial ctDNA testing is a promising dynamic tool to assess clonal evolution and predict progression in aUC pts.