Tight junctions (TJs) are the most apical component of the epithelial junctional complex forming a belt-like structure at the cellular junction. When visualized by freeze-fracture electron microscopy they appear as a branched network of intramembrane strands that correspond to the sites of direct membrane contacts and that are composed of the integral membrane claudin proteins. The TJs act as a primary barrier to the diffusion of solutes through the paracellular space (barrier function) (Tsukita et al., 2001). They also prevent the intermixing of intramembrane proteins and lipids and thus create a boundary between the apical and the basolateral membrane domains of polarized epithelial cells (fence function) (Tsukita et al., 2001). Interestingly, the fence function seems not to depend on TJ strands (Umeda et al., 2006). Recents evidence indicates that the TJs also participate in signal transduction mechanisms which regulate cell proliferation and morphogenesis (Matter and Balda, 2003; Matter and Balda, 2007). This module describes the major molecular interactions responsible for the formation of TJ strands and for the rectruitment of the PAR-3-PKC-PAR-6 and CRB3-Pals1-PATJ complexes that function in tight junction formation (Ebnet, 2008).