In recent years droplet microfluidics has become a quickly evolving research field. The availability of a wide range of technologies for droplet generation and manipulation has enabled the applications of droplet microfluidics in a wide variety of fields, from single cell analysis to material synthesis. In this review we summarize the main technologies for droplet microfluidics and discuss the recent advances in technologies that enable droplets as microreactors with complete functions. Applications of microdroplets in chemical reactions, particle synthesis, and single cell analysis are also briefly reviewed.