• Able to understand and willing to sign a written informed consent document.

• Able to read and write in English.

• Must be eligible to receive nivolumab (Cohorts A-1, B-1) or pembrolizumab (Cohorts A-2, B-2) singly or in combination with other FDA-approved agents (TKIs or chemotherapy) according to standard of care practices, as determined by the clinical judgment of the investigator.

• Prior and concurrent therapy criteria

• o Patients should either be ICI-naïve (Cohorts A-1, A-2) or be currently receiving adjuvant or front-line PD-(L)1 based therapy singly or in combination with FDA-approved agents (TKIs or chemotherapy) (Cohorts B-1, B-2).

• Locally advanced or advanced/metastatic solid tumor for which nivolumab OR pembrolizumab is on-label.

‣ NOTE: IV nivolumab is FDA-approved in the following indications: RCC, melanoma, NSCLC, SCCHN, UC, dMMR/MSI-H CRC, HCC, esophageal cancer, and gastric, gastroesophageal and esophageal adenocarcinoma (gastric/GEJ).

⁃ NOTE: IV pembrolizumab is FDA-approved in the following indications: RCC, melanoma, NSCLC, SCCHN, UC, dMMR/MSI-H CRC, HCC, esophageal cancer, gastric/GEJ, cervical cancer, cutaneous squamous cell carcinoma (cSCC), Merkel cell carcinoma (MCC), endometrial carcinoma, tumor mutational burden-high (TMB-H) cancers, triple negative breast cancer (TNBC).

• Cohort-specific criteria.

‣ Cohort A-1: Patients who are treatment-naive (i.e. for whom nivolumab is planned but has not yet been initiated) are eligible to enroll.

⁃ Cohort B-1: Patients who are already receiving treatment with nivolumab (singly or in combination with TKI or chemotherapy) OR a different ICI-therapy but are willing to switch to nivolumab monotherapy or nivolumab based combinations may eligible to enroll if nivolumab is on-label for their cancer.

⁃ Cohort A-2: Patients who are treatment-naive (i.e. for whom pembrolizumab is planned but has not yet been initiated) are eligible to enroll.

⁃ Cohort B-2: Patients who are already receiving treatment with pembrolizumab (singly or in combination with TKI or chemotherapy) OR a different ICI-therapy but are willing to switch to pembrolizumab monotherapy or pembrolizumab based combinations may eligible to enroll if pembrolizumab is on-label for their cancer.

⁃ NOTE: Patients who are currently receiving nivolumab + ipilimumab combination as induction may be eligible to enroll in Cohort B-1 following induction (i.e. during planned maintenance) in indications including but not limited to advanced/metastatic melanoma, ccRCC, MSI-H/dMMR mCRC.

⁃ NOTE: Patients for whom nivolumab + ipilimumab combination is planned as maintenance are not eligible (i.e. NSCLC patients being treated per CheckMate-227 or CheckMate-9LA).

⁃ NOTE: Patients for whom anti-PD-1 based immunotherapy is planned as neoadjuvant therapy are not appropriate. Such patients may be considered for enrollment at the time of commencing adjuvant therapy in cohorts A-2 or B-2 as appropriate.