Microplastics are defined as plastic fragments with dimensions less than 5 mm, which are gaining much attention due to their ubiquitous and possibly dangerous presence in marine environment, moreover also synthetic fibres, coming from washing processes, which are released every year in oceans. Microplastics and microfibers represent a threat for marine ecosystems, and consequently for humans, since they may be ingested by fauna, adsorb persistent organic pollutants and leach toxic additives and seas. Moreover, they can act as a vector of toxic compounds such as heavy metals, persistent organic pollutants (POPs) and polychlorobiphenyls (PCBs), enhancing their biotoxicity. There is little information about the presence of microplastics and synthetic microfibers in aquaculture. For this reason, the purpose of this special issue is to collect papers on the presence of microplastics and synthetic fibers in species reared in aquaculture in order to have a more precise picture of this type of contamination in marine organisms reared and consumed daily by humans.