Vaccine safety is a key element for public trust in vaccines and, as such, vital to the success of vaccination programmes. This chapter provides an overview of all these issues for nurses, midwives and allied healthcare professionals in order to support their work and inform their conversations with people about the vaccines that they may require. This chapter will consider the following:  • How vaccines work  • Various types of vaccine/vaccine platform  • How vaccines are tested within the different phases of trials including ongoing surveillance and licensing  • Vaccine constituents