Integrins are heterodimeric transmembrane receptors regulating cell-cell and cell-extracellular matrix interactions. Of the 24 integrin heterodimers identified in humans, a9ß1 integrin is one of the least studied. a9, together with a4, comprise a more recent evolutionary sub-family of integrins that is only found in vertebrates. Since a9 was thought to have similar functions as a4, due to many shared ligands, it was a rather overlooked integrin until recently, when its importance for survival after birth was highlighted upon investigation of the a9 knockout mouse. a9ß1 is expressed on a wide variety of cell types, interacts with many ligands for example fibronectin, tenascin-C and ADAM12, and has been shown to have important functions in processes such as cell adhesion and migration, lung development, lymphatic and venous valve development, and in wound healing. This has sparked an interest to investigate a9ß1-mediated signaling and its regulation. This review gives an overview of the recent progress in a9ß1-mediated biological and pathological processes, and discusses its potential as a target for cancer diagnosis and therapy.