The amino terminal domain (ATD) of ionotropic glutamate receptor (iGluR) subunits resides at the extracellular region distal to the membrane. The ATD is structurally and functionally the most divergent region of the iGluR subunits. Structural studies on full-length GluA2 and the ATDs from three iGluR subfamilies have shed light on how the ATD facilitates subunit assembly, accommodates allosteric modulator compounds, and controls gating properties. Here recent developments in structural and functional studies on iGluR ATDs are reviewed.