We present a detailed description of the drift chambers used as an active target and a tracking device in the NOMAD experiment at CERN. The main characteristics of these chambers are a large area (3×3 m2), a self-supporting structure made of light composite materials and a low cost. A spatial resolution of 150 μm has been achieved with a single hit efficiency of 97%. 29.40.Cs; 29.40.Gx; 07.05.Kf; 13.15.+g Drift chambers; Spatial resolution; Tracking; Neutrino oscillations [Copyright &y& Elsevier]