Although the qualitative influence of mountains over the atmosphere has been known for a long time, numerous deficiencies, linked to orography, are still noted, either in forecasts by regional models, or in the long-term behavior of climate models. This is why the French and Spanish weather services are undertaking an important field campaign to document the dynamic modifications to the atmospheric flow generated by the Pyrenean range during a 2-month period (October and November 1990) with six intensive observation periods (IOPs) of 2 to 3 days. The experimental strategy is based largely on mesoscale numerical-model results and will help to validate these models. The main focus is on the documentation of clear-air turbulence generated either by breaking mountain waves, by surface roughness, or by the wind shear induced by the lateral-flow deviation around the mountain. Experimental means include several networks of surface stations, radio soundings, constant-level balloons, four wind profilers, and several research aircraft.