The aim of this chapter is to examine the specific effects of exogenous and endogenous heat on embryonic growth during different stages of incubation, with an emphasis on controlled incubation temperature studies. The chapter explores the challenges inherent in establishing a temperature recommendation for incubation of poultry eggs, and considers forms of pre-incubation holding as well as the outcomes of both early and late incubation temperatures. Finally, the chapter addresses the connection between incubation temperatures and poultry immunology, as well as thermal manipulations and the potential impact on moderating post-hatch heat stress in chicks. Suggestions on where to find further information is provided as well as suggestions for future lines of research in this area.