Peyer's patches are lymphoid organs situated on the anti-mesenteric side of the mid-intestine. Within the Peyer's patches, immune responses to intestinal-derived antigens are initiated. In this chapter, we will discuss the structure and function of the Peyer's patches. In addition, the development of Peyer's patches during fetal life will be reviewed, with an emphasis on the reciprocal interaction between distinct hematopoietic cell subsets and their stromal environment in the Peyer's patch anlage.