Background <p>Metastatic urothelial carcinoma (mUC) exhibits pronounced molecular heterogeneity between primary tumors and metastases, particularly in terms of drug targets such as NECTIN4, FGFR3 mutations, and PD-L1 expression. These differences complicate the reliable prediction of treatment response and limit the effectiveness of biomarker-driven therapy.</p> Objective <p>This article summarizes the role of pretreatment metastatic biopsies to improve precise molecular characterization and treatment selection in mUC.</p> Materials and methods <p>This work comprises a&#xa0;review of the current literature assessing molecular alterations and heterogeneity in metastatic tumor tissue, focusing on NECTIN4 expression, FGFR3 mutations, and immune phenotypes.</p> Results <p>Membranous NECTIN4 expression frequently decreases during metastasis and correlates with resistance to enfortumab vedotin. The FGFR3 mutation status can change between the primary tumor and metastases. The PD-L1 status and immune phenotypes differ between primary and metastatic tumors, with only the latter predicting the response to immunotherapy.</p> Conclusion <p>Pretreatment metastatic biopsies are essential to capture molecular heterogeneity and clonal evolution, enabling precise treatment decisions. Circulating tumor DNA and molecular imaging may complement diagnostics in the future. Robust biomarker-based clinical trials should utilize representative metastatic tumor material.</p>