A profound understanding of the immunological processes underlying vaccine responses is essential for developing improved and novel vaccines and defining optimal vaccination recommendations. Explaining these immunological concepts can also help to overcome vaccine hesitancy and improve the uptake of vaccines. Vaccines exploit the concept of immunological memory and induce specific immune responses to the vaccine antigens without causing the disease. Upon contact with the pathogen later on, immunological effector mechanisms, such as antibodies, are already in place, and memory T and B cells can react quickly and effectively, protecting against disease. The duration of protection varies significantly between different vaccines, but can be many years or even lifelong. In addition to understanding the biological basis of vaccination, evaluating how vaccine success is measured is crucial. During vaccine development, immunogenicity is assessed by quantifying specific antibody and T cell responses. Metrics such as efficacy and effectiveness provide insights into how well vaccines prevent disease in controlled clinical trials versus real-world settings. Furthermore, the safety of vaccines is rigorously monitored through clinical trials and post-marketing surveillance to ensure that they are safe for public use. This chapter covers the immunological mechanisms of vaccine responses, the principles underlying vaccine immunology, and the various factors that contribute to their success.