This chapter reviews the immune system and describes the current knowledge of its development during fetal life. It summarizes the capacity of the fetal immune system to respond to infectious pathogens and focuses on congenital infections. In tissues, immune cells like macrophages and dendritic cells (DCs) express specific receptors, such as toll-like receptors (TLRs), allowing them to recognize molecules called pathogen-associated molecular-patterns (PAMPs) that are specifically expressed by pathogens. The immune system develops during fetal life in order to be ready to control infectious microorganisms to which the infant will be exposed after birth. Immune effector functions are required to control pathogens but may lead to inflammatory responses that are potentially harmful to the fetus and to the placenta. Recent studies of the immune responses to some pathogens infecting the fetus have indicated that fetal T lymphocytes can develop effector functions similar to those of older children or adults.