Of the major mesozooplankton predators in high-latitude oceans, pelagic amphipods are amongst the least studied groups. Hyperiid amphipods represent a group of exclusively pelagic species of which many co-evolved with soft-bodied zooplankton groups such as salps and hydromedusae, using them as substrate, for food, shelter or reproduction. Amphipods of the hyperiid genus Themisto Guérin, 1825, currently comprising seven distinct species, have so far been neglected in ecological and molecular studies despite their role as key players in temperate and cold-water pelagic ecosystems. In some areas they represent the base of entire food webs and the major prey for many commercially important fish and squid stocks. Themisto amphipods are major predators of meso- and macrozooplankton in several regions from shelves to open ocean and the poles to the subtropics. In the northern high latitudes there are indications that range shifts are occurring and this is also expected in the warming waters of the Southern Ocean, highlighting the urgent need to gather the current state of knowledge and identify the many knowlegde gaps. This will allow us to reliably evaluate, monitor and predict the consequences of potential distributional and ecological shifts. We discuss distributional patterns, life history traits, feeding habits and role in regional food webs and biogeochemical cycles of Themisto amphipods and point out some key questions and hypotheses on their ecology and biology that so far remain unanswered or untested. Moreover, we will present the first molecular results on Themisto species from the polar oceans, and discuss their genetic structure and historical demography in the light of their evolutionary ecology. Finally, we will complement this with some behavioural observations and experimental data on feeding experiments carried out on Themisto species from both hemispheres.