This book deals with the concept of isosterism, bioisosterism and retroisosterism. Specifically it gives the bioisosteric correlation of tetrazole moiety with carboxylic group with the help of different parameters and theories. Carboxylic group is chiefly presents in proteins and peptides, thus emphasis is laid on the fact that how this carboxylic acid has been replaced by the tetrazoles in development of new chemical entities which are under clinical trials and others which are prevailing as drugs in market. Effective results in pharmacokinetic and pharmacodynamic properties of compounds have been observed. This book also highlights the effect of tetrazoles in place of carboxylic groups, studied specifically as templates in growth hormone, as in anti-HIV agents and as modifier of enzyme activity. Lastly it gives the idea of retroisosterism which is based on the inversion of a determined functional group present in the lead compound structure. Furthermore, the role of bioisosters in drug development and drug discovery is briefly discussed along with the future prospects.