It gives me immense academic satisfaction to present this comprehensive textbook titled Active Pharmaceutical Ingredients and Excipient Sciences, authored specifically for undergraduate students of Pharmacy (B.Pharm, VI Semester) adhering to the elective syllabus. Over the past two decades, pharmaceutical formulation development has transitioned from an empirical, art-driven discipline to a highly predictive, science-based technological domain governed by Quality by Design (QbD) and stringent international regulatory standards. An Active Pharmaceutical Ingredient (API) cannot reach its therapeutic site of action without the precise, functional cooperation of excipients. Far from being simple 'inert vehicles', modern excipients serve as essential molecular partners that modulate solubility, dissolution kinetics, mechanical stability, cellular uptake, and shelf-life. This textbook bridges fundamental chemical principles with real-world manufacturing practices, quality assurance methodologies, impurity profiling, and global regulatory compliance