Provided are antibodies, or an antigen-binding fragment thereof, that binds human IL-21, These antibodies are useful in immunoassays of IL-21 levels, and/or in vivo, ex vivo or in vitro immunochemical and other imaging methods for determining the presence of IL-21 and/or quantifying IL-21 levels, and for diagnostic, prognostic and predictive purpose, and or optimizing therapeutic regimens in patients in which IL-21 signaling is implicated in pathogenesis.