The prevention and control of infectious diseases of sheep can be achieved in a number of ways, including disease screening programmes, implementation of biosecurity measures, chemotherapeutics and vaccination. This chapter describes common diseases of sheep and some commonly used vaccines, and explains why the identification of immunological correlates underpins vaccine design. Focussing on ovine enzootic abortion (OEA) as a case study, the chapter reviews the current capability to study immunology in sheep, discusses barriers to progress and identifies future opportunities for applications of immunology to improve sheep production.