Chemical signals are essential for organisms living in aquatic environments and are likely used by every aquatic organism at some point in their life cycle. Although microfauna remains understudied in this regard, they comprise a significant portion of global biodiversity and can be extremely influential in ecosystem processes. This group of influential organisms includes parasites that attach to the inside (endoparasites) or outside (ectoparasites) of their host’s body and can have a variety of effects on individual hosts and ecosystems. Ectoparasites typically have free-living stages where they must rely heavily on chemical cues to navigate complex environments in order to locate hosts for infestation, conspecifics for mating and suitable habitat, while avoiding predators and other hazards. Additionally, some endoparasites experience free-swimming stages where they similarly rely on chemical signals. However, chemosensory behaviour has only been investigated for a small subset of aquatic parasites. This chapter summarises the research that has been conducted on the chemosensory behaviour of aquatic ectoparasites and free-swimming stages of endoparasites, including their ability to detect chemical cues and how this chemical detection proficiency influences their behaviours at key points in their life cycle. A brief background of commonly used methodology used in chemosensory research is also provided. Finally, this chapter identifies the gaps in knowledge that exist and directions that future studies should prioritise.