The aim of this chapter is to provide a comprehensive overview of vaccination strategies in broiler production, focusing on the factors that influence their implementation and effectiveness. To this end, it integrates current knowledge on the development of the avian immune system, pathogen epidemiology, vaccine technologies, and management practices. The chapter begins by examining the ontogeny and maturation of the avian immune system, from embryogenesis through the early post-hatch period. It then explores the principal infectious diseases targeted by vaccination and reviews immunization practices and approaches used in both fast- and slow-growing broilers, including those raised under alternative production systems, such as on-farm hatching. A detailed description of conventional and new generation vaccines is provided, along with the main delivery methods used in commercial settings. The chapter also discusses the multifactorial determinants of vaccine performance - including host-, pathogen-, vaccine-, and environment/management-related factors - and highlights key issues, such as immunosuppression, vaccine interference, and administration errors.