Multi-year, detailed, and comprehensive measurements, extending from the atmosphere through the sea-ice and into the ocean of the central Arctic Basin are needed to improve our understanding and modeling of the changing Arctic climate and weather, and enhance Arctic sea-ice predictive capabilities. The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) initiative aims to address this fundamental need through cross-cutting, observational and modeling activities. The program is organized around the central science question: “What are the causes and consequences of an evolving and diminished Arctic sea ice cover?” Scientific emphasis will focus on processes that transfer heat, moisture, density, momentum, and nutrients through the Arctic climate system. To address the science objectives, the program will include an intensive observational component designed to provide a process-level understanding of interdependent atmosphere, sea-ice, ocean, and biological processes that are leading to, and responding to, drastic changes in the sea-ice. Observations will be made from a manned, transpolar drifting observatory, wherein an ice-hardened ship will serve as a central hub for comprehensive, interdisciplinary observations over 1-2 years’ time. Information on spatial variability and heterogeneity in the system will be obtained using a coordinated network of distributed measurements from buoys, unmanned aerial systems, autonomous underwater vehicles, additional ships, aircraft, and satellites. A hierarchy of modeling activities will capitalize on these observations to study detailed climate processes, evaluate and improve model parameterizations, facilitate regional model intercomparisons, and elucidate the impacts of Arctic processes on hemispheric circulation patterns. The International Arctic Science Committee is helping to facilitate and coordinate this international activity.