During embryogenesis differentiation of cells originally derived from the single-celled zygote results in the emergence of an organism with highly complex humoral and cellular immune systems. In some species, particularly the mouse and the chicken, detailed descriptive and experimental analysis of the elements and interrelationships of the complex process of immunologic differentiation have been possible. Such studies have given rise to the original concept of a two-component immune system, one for humoral and the other for cellular immune reactions, each with its separate central organ, e.g., the bursa of Fabricius and the thymus in the chicken, respectively. The study of the ontogeny of immunity in man is still in its infancy and currently limited largely because of legal and moral restraints. However, through the use of tissue culture and other sensitive in vitro techniques, human immunologic embryology has recently emerged as a provocative and scientifically exciting endeavor. The following summarizes salient aspects of our current knowledge of the development of cellular and humoral immunity in the human fetus.