Systems and methods for controlling fluids in microfluidic systems are generally described. In some embodiments, control of fluids involves the use of feedback from one or more processes or events taking place in the microfluidic system. For instance, a detector may detect one or more fluids at a measurement zone of a microfluidic system and one or more signals, or a pattern of signals, may be generated corresponding to the fluid(s). In some cases, the signal or pattern of signals may correspond to an intensity, a duration, a position in time relative to a second position in time or relative to another process, and/or an average time period between events. Using this data, a control system may determine whether to modulate subsequent fluid flow in the microfluidic system. In some embodiments, these and other methods can be used to conduct quality control to determine abnormalities in operation of the microfluidic system.