This special issue contains four review articles that analyze the development and biology of innate lymphoid cells (ILCs), which are the most recently-discovered group of innate immune cells. This unique group of lymphoid cells lacks the RAG gene and consequently does not express B cell nor T cell antigen-specific receptors. They are abundant at mucosal surfaces, where they play a role in immunity and homeostasis. The ILCs are the focus of intensive research efforts to understand their development and function.