The Ikaros family of proteins – comprising Ikaros, Aiolos, Helios, Eos and Pegasus – are zinc finger transcription factors. These proteins participate in a complex network of interactions with gene regulatory elements, other family members and a raft of other transcriptional regulators to control gene expression including via chromatin remodelling. In this way, Ikaros family members regulate important cell-fate decisions during hematopoiesis, particularly in the development of the adaptive immune system. Mutation of several family members results in hematological malignancies,especially those of a lymphoid nature. This review describes the key roles of Ikaros proteins in development and disease, their mechanisms of action and gene targets, as well as explaining their evolutionary origins and role in the emergence of adaptive immunity.