The book presents a systematic exploration of the features and implications of external economies of scale. It examines how external economies of scale may affect resource allocation, income distribution, international trade, international factor movements, and national welfare of an economy. It synthesizes existing theories and results in the literature and presents new analytical techniques and resuts using a unified framework, which helps the readers follow the logic and derivation of the arguments and propositions. Exercises are provided at the end of each chapter to help the readers get familiar with the techniques presented in the chapter and explore new applications and results. The book is thus suitable for graduate students, researchers and policy makers who are interested in the latest developments in the field.