The present invention generally relates to systems and methods for the control of fluidic species and, in particular, to the coalescence of fluidic droplets. In certain instances, the systems and methods are microfluidic. In one aspect, the invention relates to systems and methods for causing two or more fluidic droplets within a channel to coalescence. The fluidic droplets may be of unequal size in certain cases. In some embodiments, a first fluidic droplet may be caused to move at a first velocity, and a second fluidic droplet may be caused to move at a second velocity different from the first velocity, for instance, substantially greater than the first velocity. The droplets may then coalesce, for example, upon application of an electric field. In the absence of an electric field, in some cases, the droplets may be unable to coalesce. In some cases, two series of fluidic droplets may coalesce, one or both series being substantially uniform. For instance, one series of droplets may have a distribution of diameters such that no more than about 5% of the droplets have a diameter greater than about 10% of the average diameter. In certain cases, one or more series of droplets may each consist essentially of a substantially uniform number of entities of a species therein (i.e., molecules, cells, particles, etc.). The fluidic droplets may be coalesced to start a reaction, and/or to stop a reaction, in some cases. For instance, a reaction may be initiated when a species in a first droplet contacts a species in a second droplet after the droplets coalesce, or a first droplet may contain an ongoing reaction and a second droplet may contain a species that inhibits the reaction. Other embodiments of the invention are directed to kits or methods for promoting the coalescence of fluidic droplets.