Research on the ecological significance of smaller copepod species in the Arctic has not received adequate attention. Possible reasons are their smaller size, fragile bodies, difficult handling and identification and that they are frequently underrepresen ted due to undersampling by coarse plankton nets.Although smaller in size, these species are of major importance with regard to abundance, secondary production and energy transfer through the arctic food web. Pseudocalanus spp Boeck 1872 are common and abundant in Svalbard and Norwegian coastal waters. Having a circumpolar distribution, the genus consists of several frequently co-occurring sibling species which are often pooled together and treated as Pseudocalanus spp.-or even Paracalanus/Pseudocalanu s-in the existing literature, thus ignoring species-specific differences in habitat, fysiology and life history. A lack of interspecific morphological differentiation and potential co-occurrence of cryptic species has plagued the scientific study of the g enus and it is confirmed that interspecific divergence in morphological, meristic, and morphometrical features is unusually weak in Pseudocalanus. In order to advance our understanding of the species within Pseudocalanus it is crucial to have a reliable m ethod of identification.A recent effort combined and compared morphological and species-specific Polymerase Chain Reaction (PCR) identification. The results were promising, albeit inconsistent and could indicate populations with varying degrees of isolat ion, different origin, hybridization or additional species. It is paramount to further develop this technique until a solid method of identification is established. The taxonomic information will be related to distribution and ecological adaptations, th e origin of the populations (local or advected), environmental preferences (temperature, food), life history, evolution and potential population effects of a changing climate.