This comprehensive fourth edition of Stephen P. Radzevich’s acclaimed work presents a thorough exploration of gear theory, offering both theoretical foundations and practical applications for engineers and researchers working with gear design and manufacturing.The fourth edition has been meticulously revised with the entire manuscript thoroughly updated to enhance clarity and precision. Many illustrations have been completely replaced with superior versions, and numerous new illustrations have been added throughout the text. The equivalent pulley‑and‑belt transmission concept has been significantly developed and refined, and the author has incorporated an important new result in envelope theory – a contribution valuable not only to gear theory but also to pure mathematics in the field of differential geometry of surfaces. Additionally, this edition includes the latest advances in gear theory from the beginning of the twenty‑first century, ensuring readers have access to cutting‑edge developments in the field.This book is essential reading for mechanical engineers, automotive engineers, manufacturing engineers, researchers, and graduate students seeking to master the science of modern gear design.