The present invention generally relates to microfluidic devices, including systems and methods for tagging droplets within such devices. In some aspects, microfluidic droplets are manipulated by exposing the droplets (or other discrete entities) to a variety of different conditions. By incorporating into the droplets a plurality of nucleic acid “tags,” and optionally ligating then nucleic acids together, the conditions that a droplet was exposed to may be encoded by the nucleic acid tags. Thus, even if droplets exposed to different conditions are mixed together, the conditions that each droplet encountered may still be determined, for example, by sequencing the nucleic acids.