Tight Junctions are the most apical element of the junctional complex in epithelial and endothelial cells. Tight Junctions form a barrier to paracellular movement of substances separating the apical and basolateral fluid compartments on opposite sides of the epithelial cell layer. The Claudin family are Tight Junction proteins expressed in both endothelial and epithelial cells. They participate in the development of tissue barriers between different tissue compartments by regulating the efflux of molecules through Tight Junction complexes. At least 24 different Claudin members are known today, all of which are thought to vary in expression depending on location and cell type. Relatively little is know about Claudins and their role in carcinogenesis and progression to metastasis. Recently, this new area of research has become very promising as a result of the frequent existence of altered Claudin expression in cancer. That Claudins are pivotal in the maintenance of Tight Junctions function begs investigation into the changes that can occur during the metastatic process.