A Study of 68Gallium PSMA-PET/CT Scans in People With Bladder Cancer or Skin Cancer

Inclusion Criteria for Urothelial Carcinoma Cohort: * Patients with histologically confirmed metastatic urothelial carcinoma with extrapelvic nodal and/or visceral sites of disease (including lung, liver, bone, or soft tissue) * At least 1 lesion assessable by FDG PET/CT according to RECIST and PERCIST guidelines where applicable, that are determined suspicious for metastasis by an MSKCC attending radiologist or nuclear medicine physician. * Karnofsky performance status ≥50% (or ECOG/WHO ≤2) * Participant is ≥18 years of age * Patient must be able to understand and is willing to sign a written informed consent document Inclusion Criteria for Melanoma Cohort: * Patients with histologically confirmed metastatic melanoma * At least 1 metastatic lesion assessable by CT or FDG PET/CT according to RECIST that is determined to be suspicious for metastasis by an MSKCC attending radiologist or nuclear medicine physician * ECOG \<= 2 * Participant is \>= 18 years of age * Patient must be able to understand and is willing to sign a written informed consent document Exclusion Criteria for Urothelial Carcinoma Cohort: * Patients with pelvic node-only metastatic disease. If the patient has lymph node only disease, at least one PET-assessable node must be located outside of the pelvis * Patients with bone only disease * Unable to lie flat, still, or to tolerate a PET scan * Patient undergoing active treatment for non-urothelial malignancy, other than skin basal cell or cutaneous superficial squamous cell carcinoma that has not metastasized. * Patients on a therapeutic clinical trial where PSMA imaging would interfere with the conduct of the trial * Patients undergoing active surveillance with a known history of non-urothelial malignancies * Women who are pregnant. All women of childbearing potential must have a documented negative pregnancy test. Exclusion Criteria for Melanoma Cohort: * Unable to lie flat, still, or tolerate PET scan. * Patient is on another therapeutic trial where PSMA imaging would interfere with the conduct of the trial. * Women who are pregnant. All women of childbearing potential must have a documented negative pregnancy test.