Long-term treatment in the setting of metastatic Merkel cell carcinoma (MCC) and urothelial carcinoma (UC) has shown that current first-line chemotherapeutic agents are losing effectiveness and that there are limited treatment options available outside of radiation therapy and surgical interventions. The use of immunotherapeutic agents such as monoclonal antibodies has been considered a promising alternative for cancers that progress despite treatment with radiation therapy, surgery, and/or chemotherapeutic agents. Cancer cells escape immune surveillance by interrupting immune checkpoint pathways, resulting in dysregulation of T-cell function and so preventing its antitumor effects. In early 2017, avelumab (BAVENCIO®), a PD-L1-blocking monoclonal antibody agent, was approved for the treatment of metastatic MCC and UC. Trials that evaluated avelumab for the treatment of metastatic MCC and UC were the JAVELIN Merkel 200 Trial and the JAVELIN Solid Tumor trial, respectively. Efficacy results for both trials showed positive overall response rate (ORR) and progression-free survival rate (PFS). A strong safety profile was also established for avelumab. This review provides a brief introduction to checkpoint inhibitors and focuses on the recently approved PD-L1 inhibitor, avelumab.