JASIN was designed to observe the physical processes causing mixing and to quantify aspects of the heat and momentum budgets in mid-latitude oceanic and marine atmospheric boundary layers. The multiplicity of processes to be sampled necessitated a large experiment, and JASIN involved 14 ships and 3 aircraft, with more than 50 teams of investigators from 9 countries. The experiment took place from July to September 1978 in the North Rockall Trough, an area of deep water several hundred kilometres off the west coast of Scotland. Hydrographic surveys revealed eddies 100 km across, and intensive measurements on smaller scales revealed fronts in the confluence zones of the eddy field. The development of the mixing layer and seasonal thermocline, internal and inertial wave fields, and the Ekman response to winds are all complicated by the fronts and eddies. The atmospheric boundary layer on scales of 200 km was non-steady and inhomogeneous. Surface-generated turbulence occupied the subcloud layer and transfers through the cloud layer occurred on scales greater than 50 km or intermittently. Possible driving factors were the variability in surface fluxes near the oceanic mesoscale eddies, radiative processes in the cloud layer and condensation processes. The lifting of warm sector boundary layer air from widely differing sources into the free atmosphere at a front resulted in well mixed moist layers that exhibited near-thermal wind equilibrium.