NGAL, also known as lipocalin 2, LCN2, and siderocalin, is a small molecule composed of 178 amino acids. It belongs to the lipocalin superfamily of 20 structurally related secreted proteins. Members of the lipocalin family share a highly conserved fold with an eight stranded antiparallel beta barrel, and act as a transporter, carrying small molecules to specific cells. NGAL occurs predominantly in a monomeric form, with a small percentage occurring as a dimer or trimer. It was originally identified as a 25 kD protein covalently bound to gelatinase from human neutrophils. It is a critical component of innate immunity because its ability to bind iron-siderophore complexes limits bacterial growth. Human NGAL is expressed at very low levels in human tissues, including kidney, trachea, lungs, stomach, and colon, and its expression increases greatly in the presence of inflammation and injured epithelia. In addition to its role in innate immunity, it is a promising biomarker for acute kidney injury that is beginning to be used in clinical practice in addition to research studies.