The determination of related compounds is emerging as a very important issue in the field of pharmaceutical analysis. It requires highly specific and accurate methods of analysis to be able to resolve the interferences that may arise during the analysis of these compounds. The present book deals with the term "Related" as a general term including different types of relationships between the studied drugs. The book is devoted to the pharmaceutical applications of different analytical methods for the determination of a mixture of two related drugs or the determination of certain drugs in presence of other related ones. Several drugs belonging to different pharmacological and chemical groups were determined throughout the present study, namely, a mixture of glibenclamide and metformin hydrochloride, bopindolol malonate, celiprolol hydrochloride, 2-amino-4-chlorophenol (a potential impurity of chlorzoxazone), and atenolol. The proposed methods were successfully applied for the determination of the studied compounds in pharmaceutical formulations and/or biological fluids.