This book introduces a unique 3D printing method that prints metal parts by ejecting metal micro-droplets: a low-cost, contactless, and environmentally friendly 3D printing technology. This book follows a deductive approach to describe the fundamentals of metal droplet-based 3D printing and reveal the relationships and unique features among multiple specific techniques used in droplet-based 3D printing. It starts with a general description of the principles and techniques involved in this technology and then focuses on the details of several specific metal droplet-ejection methods. Next, it puts forward various specific 3D printing techniques and illustrates their applications. This book is a valuable reference for scholars and researchers who work on metal 3D printing and other related areas. It is also used as a textbook for college graduate courses in mechanical manufacturing and material processing.