Global change reaches the Arctic via the exchange of heat and moisture by winds and currents between the equator and the poles. Winds are relatively unobstructed by land and move air and moisture north or south at locations all around the Arctic. Currents are confined by continents to a few pathways. There are only two routes for currents leaving the Arctic, via the Canadian Archipelago or via Fram Strait (east of Greenland). The former is the focus of the Canadian Arctic Through-flow (CAT) study during the IPY. Exchanges via currents between the Arctic and adjacent oceans contribute to balancing the budgets of fresh-water and heat. Knowledge and understanding of these exchanges falls short of the needs for predicting climate change impacts in the Arctic. At present the science is at an exploratory stage, requiring field measurements to reveal baseline values, their variation and change and to provide data for thought, analysis and interpretation. Efforts using theory to simulate observations and to forecast future change will gain prominence as the science progresses. The objectives of the Canadian Arctic Through-flow study are to acquire the first accurate measurements of flow through Canadian Archipelago, to analyze these observations and to develop understanding of likely change with climate. In the CAT study we are developing computer simulation models of the Canadian Arctic to facilitate the latter task. The models will be used to study oceanic aspects of Arctic variability and change. The Arctic Ocean is connected to the Atlantic via shallow straits. The narrow shallow sections of straits on the Canadian polar shelf are the best places to measure through flow. However, there are intimidating logistic and environmental challenges in this northern sea – remoteness, harsh climate, persistent heavy pack ice, icebergs and strong tides. One unique oceanographic challenge in the Canadian Arctic is the proximity of the geomagnetic pole, which seriously compromises the value of compasses for measuring current direction.