The IKZF family of transcription factors are essential regulators of lymphopoiesis. Ikaros, Helios, Aiolos and Eos function as transcriptional repressors and activators during T and B cell differentiation and in mature cell function, depending on the stage of development and/or cell type. Their potential mechanisms of action are varied. Ikaros family proteins partner with multiple complexes, including NuRD, PRC2 and transcription elongation factors, to modulate gene expression and the chromatin state. In humans, mutations in the IKZF genes are associated with B cell deficiency, leukemias and autoimmunity. In this review, we focus on the function of Ikaros family proteins in early T and B lymphocyte development, and discuss the molecular and physiological activities of this family.