Controlling the size and the monodispersity of droplets is an important task in droplet-based microfluidics. A closed-loop control mechanism with actuation and sensing of droplet size is ideal for systems that require a high degree of monodispersity. Most droplet-based microfluidic systems control the size of droplets using the flow rates, applied pressures, and flow rate ratios. These concepts are based on the hydrodynamic interactions during the formation process of droplets and requires a relatively long time to stabilize. Changing flow rates and pressures requires the control of external components such as pumps and pressure controllers. This chapter discusses in situ active control of the formation process. The advantage of this approach is that the control signal in the form of a voltage or a current can be induced into the system allowing immediate response. The control schemes are categorized and discussed according to their actuation concepts: thermal and magnetic actuation.