Polar ecosystems span a vast range of contrasting environments from open ocean to multi-year sea-ice and from dry lakes to glaciers. Most of them represent extreme environments with harsh conditions inhabited by organisms that represent remarkable examples of adaptation, specialisation, and diversity. Many polar marine, freshwater, and terrestrial ecosystems currently undergo substantial changes, but our knowledge about the virgin state of these systems is scarce. In many cases, this IPY provided the first comprehensive and systematic investigations of these biological and ecological systems. Theme 3 includes the full range of microbial, invertebrate, plant, fish, avian and mammalian species interacting with their unique polar environments. Small scale, specific environments are considered as well as interconnections between large ocean and terrestrial systems.