Additives are essential for the industrial application of polymers as plastics. This book is intended as a practical introduction to plastics additives. It is focused on how their mechanism of action and chemical structure relate to performance, enabling a better understanding of how and why the current additives work, as well as market and research trends. Specific examples are included to illustrate the general observations. The range of additives covered in the book is broad, including antioxidants, light stabilizers, heat stabilizers, plasticizers, lubricants, processing aids, impact modifiers, antimicrobials, colorants, flame retardants, antistatic agents, fillers and reinforcements, and chemical blowing agents, among others. Each chapter is logically structured to include (a) key terms and definitions, (b) function in plastic materials, (c) main examples and their chemistry, (d) mechanism of action, (e) chemical structure - performance relationships, (f) test methods, (g) toxicological profile and safe handling, and (h) market and research trends. It is intended for a range of target groups, spanning industry, including those seeking a new role in industry (a resource for pre- and on-the-job training), and academia, including researchers, lecturers, and students (an educational tool for teaching and learning).