The work selected for the book was constructed with the principle idea for co-relating the structure of bio-active molecules and antimicrobial activity. On the basis of literature survey and biological activity reported in Medicinal Chemistry of the compounds like 4-oxo-thiazolidine, 1,2,4-triazole and 2-oxo-azetidine derivatives, we have modified the specific structural changes and extended, the search for new bio-active molecules.