Phase 4 studies, also known as post-marketing surveillance (PMS) studies, are a crucial step of drug development and are carried out after marketing approval of the drug. The objective is to obtain detailed information on the safety and effectiveness in a real-world setting of the newly approved drug or device. Since Phase 3 trials are carried out for a shorter duration in a population of few hundreds to few thousands, they may not be able to detect all adverse effects, especially those that are rare or those that occur on chronic use of the drug or device. The methodology for post-marketing surveillance is chosen based on the objectives of the study and each comes with its own share of advantages and limitations. A major proportion of Phase 4 studies are observational in nature; however, there are some clinical trial-based designs, which are performed depending on the requirements. Periodic submission of aggregate safety reports is a regulatory requirement during Phase 4 of drug development. This chapter seeks to provide an overview of post-marketing surveillance studies and their importance in drug development.