Understanding and overcoming cancer therapy resistance is a major overarching goal of our precision oncology program. One of the greatest hurdles is the lack of reliable model systems to untangle resistance on a patient to patient basis. Animal models have been widely used to test highly focused questions but lack the landscape of complexity needed to answer specific questions for patient care. Herein, we will harness the power of the patients own tumor to study their disease. We propose using patient-derived cancer organoids (PDOs) from bladder and prostate cancer patients, referred to as genitourinary (GU) cancers, as an exemplar of common cancers prone to therapy resistance and progression. PDOs are small 3-dimensional multicellular structures that can be used to study individual mechanisms of tumor formation and drug response. Unfortunately, the majority of PDOs cannot be maintained for long periods of time, limiting their use for research. The major focus of this proposal will be to interrogate different protocols currently in use with the goal of optimizing and standardizing procedures to achieve consistent, reproducible, robust PDOs for every patient. The systematic validation of growth conditions in combination with extensive genomic and functional characterization of the PDOs, and the generation of a fully annotated organoid biobank with clinically relevant information will accelerate uncovering of the molecular aspects of tumor formation and the availability of new treatment options. The establishment of a reliable PDO program will positively impact the way we study tumor resistance while simultaneously decreasing the reliance on animal models.