Introduction -- Fundamental theory of uniform metal droplet ejection and deposition -- Equipment for uniform droplet ejection -- Ejection of continuous uniform droplet stream and its control -- Drop-on-demand ejection of uniform metal droplet and its control -- Uniform solder droplet deposition and its 3D printing technology -- Uniform aluminum droplet ejection and deposition and their controlling techniques -- Microstructure evolution of uniform aluminum droplets during 3D printing -- Application prospect of 3D printing based on uniform metal droplet ejection.