While the Arctic Ocean is changing rapidly, we have barely begun to understand the dynamics of animal life under the permanent sea ice, as it exists today. During the MOSAiC expedition, RV Polarstern was moored to an ice floe and drifted more than 3,000 km across the central Arctic Ocean, enabling multidisciplinary observations of the inter-linked processes in atmosphere, sea ice, ocean and ecosystem. We studied year-round changes in diversity, abundance, vertical distribution, physiology, and ontogeny of Arctic ectotherms from the pack ice at the surface into the deep ocean. Imaging profilers show the fine-scale distribution of zooplankton at high resolution, how the vertical distribution and aggregation of different species change with season, and how zooplankton species prepare for reproduction already in the deepest winter. Systematic sampling with nets shows that the pelagic food web was active from the under-ice habitat down to bathypelagic depths throughout the winter, supporting a variety of predators, such as amphipods, polar cod (Boreogadus saida), and the understudied diversity of jellyfish. The first-ever hydroacoustic survey of the Transpolar Drift recorded the change in pelagic biomass in time and space, and highlights brief periods of diel vertical migration in spring and autumn. Video surveys with a Remotely Operated Vehicle (ROV) documented the use of the under-ice habitat by polar cod and various jellyfishes. A new under-ice net mounted on the ROV can provide new insights in the connection of the life cycles of sympagic amphipods with the seasonal change of sea-ice- and water column properties. We collected thousands of samples for the analysis of condition, physiological parameters, food preference, microplastic, and trophic biomarkers, and conducted numerous rate process measurements including respiration, feeding and reproduction, for key species with the goal of unravelling the sources and fate of carbon in the food web. The first results demonstrate how sampling techniques from the days of Nansen in combination with modern technology can unfold a comprehensive picture of the contribution of Arctic fauna to ecosystem functioning and biogeochemical cycles, as well as their resilience and potential changes in a future seasonally ice-covered Arctic Ocean. We will present and discuss first results and conclusions emerging from our data with regard to the scientific objectives of MOSAiC.