• Be willing and able to provide written informed consent/assent for the trial.

• Age ≥ 18 years.

• Be treatment naïve and have advanced (metastatic or recurrent) pathologically proven urothelial carcinoma. Patients progressing more than 12 months of their last dose of platinum-based neoadjuvant or adjuvant chemotherapy are eligible.

• Have a performance status of 0 or 1 on the ECOG Performance Scale.

• Demonstrate adequate organ function as defined in Table 1., all screening labs should be performed within 10 days of treatment initiation.

• Be ineligible for chemotherapy in the assessment of the treating physician or refusing chemotherapy in frontline setting. Cisplatin ineligibility is defined as Cresatinine Clearance (CrCl) \< 60cc/min, NYHA class III hear failure, grade II neuropathy and grade II hearing loss, the latter two also apply to carboplatin-based chemotherapy.

• Be approved to begin treatment with pembrolizumab per standard of care and pembrolizumab must be available to the patient. Patient must enroll on this study prior to the receiving the second dose of pembrolizumab and prior to having restaging imaging after receiving pembrolizumab. Other PD1/PDL1 antibodies are now allowed.

• Have measurable disease based on RECIST 1.1. We request an OPTIONAL core biopsy from an accessible metastatic site after a minimum of 2 cycles of treatment AND prior to progression of disease to help the investigators better understand the activity of these drugs in tumor tissue.

⁃ In addition, we request an OPTIONAL collection of any surgical specimens obtained per standard of care during the study. For instance, specimens from metastasectomy while patient is undergoing therapy on this study.