The book deals with studying the quantification of some investigated pharmaceutical drugs using three different techniques. Some azo compounds involved in spectrophotometric method as reagent to form stable complex with drugs as ion-pair complex in buffer solutions of different pH values. The book deals also with the determination of investigated pharmaceutical drugs with ion–selective membrane sensors, The potentiometric method used has the advantage of application in turbid and coloured solutions and gives accurate results without any pretreatment steps. The studying deals also with the quantification of investigated drugs in various pharmaceutical preparations using regular HPLC-UV system and determination of OXO-HCl in human plasma samples using micro-capillary µHPLC-SPE-MS, the results give a highly selective, sensitive, rapid, and direct method for determination of drugs in pharmaceutical preparations and human plasma. The proposed methods can be used as alternative methods to the reported ones for the routine determination of pharmaceutical forms. This encourages their successful use in routine analysis of drugs under study in quality control laboratories.