Calix[n]arenes represent an interesting class of preorganized aromatic hosts exhibiting an enhanced ability for cation-¿ interaction. In many biological systems, metal cation-¿ interactions play an important role in molecular recognition. Interest has focused on the interaction between organic cations in both natural and synthetic receptors. Systematic studies of a series of related structures of artificial receptor molecules have revealed the role played by cation-¿ interactions. Calixarenes and related skeletons are a widely used platform in order to study cation-¿ interactions. This book emphasis on functionalization of calix[4]arene and its applications in various fields such as inclusion complex, ion selective electrodes, fluorescence sensor, liquid crystals and calix[4]arene based nanoassemblies as sensor.