Biological invasions have become a topic of concern due to their increasing occurrence and ecological and economic consequences. This chapter examines the stages, mechanisms and effects of biological invasions from a parasitological point of view, focusing on parasitic protists, microsporidians, fungi, oomycetes, helminths and arthropods. We present hypotheses and mechanisms underlying biological invasions of parasites and hosts, such as the enemy release hypothesis, spillover, spillback, disease acquisition and invasional meltdown. We illustrate these mechanisms and the effects of biological invasions on parasite–host interactions and native ecosystems with examples from aquatic environments. In terms of effects, we discuss direct competitive and consumer–resource interactions and indirect density- and trait-mediated effects by invasive parasites, ranging from the individual to the community and ecosystem level. We conclude the chapter with some remarks on management and mitigation aspects.