In the present volume, an effort has been made to organize and present a comprehensive overview of receptors for cationic guests. This begins with reviews of the various basic complexing systems: podands, crown ethers, lariat ethers, cryptands, and spherands. Natural cation-binding agents are also important hosts in supramolecular chemistry and their chemistry has been extensively studied. The structural aspects of complexation by natural hosts and crown ethers are reviewed separately and the topic is addressed for specific receptor systems within chapters dedicated to specific hosts. The physical chemistry of complexation and transport is reviewed along with the important application of anion activation, sometimes called phase-transfer catalysis. In the second dozen or so chapters are presented overviews of specialized areas such as the formation of alkalides, the development of chemical sensors, chromoionophores, and redox-switchable receptors. The volume concludes with discussions of transition metal complexes of macrocyclic ligands, second-sphere coordination, complexes of organometallics, and finally of fullerenes.