Medicinal chemistry serves as the essential bridge connecting fundamental chemical principles with biological outcomes. It provides the molecular rationale for how subtle modifications in chemical structure dictate pharmacological activity, pharmacokinetic profiles, and therapeutic efficacy. This eBook, Medicinal Chemistry (Theory), has been meticulously developed to serve as a comprehensive, self-contained study companion tailored specifically to the requirements of the fourth-semester Bachelor of Pharmacy (B.Pharm) curriculum. Rather than treating classification, organic synthesis, and pharmacology as isolated domains, this text integrates them into a single, cohesive narrative. Following the unit-by-unit structure prescribed by the syllabus, each drug class is systematically explored through its fundamental chemical classification, structure–activity relationships (SAR), mechanisms of action, and clinical applications. For every representative drug highlighted with an asterisk (*) in the syllabus, detailed synthetic schemes and key chemical transformations are provided to deepen the learner's understanding of synthetic pathways.