Visible by the naked eye Peyer’s patches are the most prominent organized lymphoid tissue in the mammalian small intestine. Taking a closer look with a microscope one might be surprised that the intestinal wall in addition to Peyer’s Patches harbours a huge number of smaller lymphoid aggregates, which are heterogeneous in size and cellular composition. These range from tiny clusters of lymphoid progenitor cells in the crypt zone, referred to as Cryptopatches, to more robust aggregations termed Isolated lymphoid follicles, that bulge out from the crypt zone and resemble Peyer’s patch follicles in aspects of architecture, cellular composition and function. Recent reports revealed that Cryptopatches develop into Isolated lymphoid Follicles in response to adequate extrinsic stimulation. Although most immunologists neglected these lymphoid structures for decades, on a whole they might contain at least as many immune cells as Peyer’s Patches and seem to be required for induction of intestinal IgA and maintenance of gut homeostasis. This chapter outlines phenotype, development and putative functions of these small solitary intestinal lymphoid structures.