Drug analysis is important in several phases of pharmaceutical development, such as stability studies, formulation and quality control. Several techniques have been employed in the development of analytical methods for drug quantification in both biological fluids and dosage forms. The latter is the simplest kind of sample due to its high analyte content and few matrix effects, so a large number of techniques and methods are suitable for drug quantification in dosage forms. The bioanalytical methods for the determination of some antidepressant drugs are quantified in biological matrix. In this book, the development and validation of UPLC-MS/MS methods for the quantification of some antidepressant drugs (paroxetine, amitriptyline and its metabolite) in human plasma are described. All the developed methods were found selective, sensitive and rapid. They have simple extraction procedure and a short run time. They were also found suitable for clinical pharmacokinetics studies in healthy human volunteers following oral administrations.