The successful development of vaccines is a lengthy process, requiring input from different expertises, such as research and development, quality control, quality assurance, production, regulatory affairs, and marketing and sales. A cornerstone in vaccine development is the availability of a panel of high-quality assays that can reduce the risk of failure in clinical trials and licensure. This review highlights the quality-control issues and approaches in vaccine development from different viewpoints: discovery, process development, assay development, clinical development and the postlicensing phase.