[en] Liquid-Argon Time Projection Chamber (L Ar-Tpc) technology provides unique features in event imaging and calorimetric resolution. These characteristics, combined with a good rejection power and a (virtually) unlimited active mass, make this technology one of the best candidates for the next generation of long-baseline neutrino oscillation experiments. The ArgoNeuT detector, a small L Ar-Tpc (0.24 on of active mass) exposed to the NuMI beam at Fermilab and currently producing the fist low-energy neutrino events in a L Ar-Tpc, is the first step of a staged US program to realize and operate a massive L Ar-Tpc detector for ν physics. This paper describes design details, experimental goals and current status of the experiment and reports a first example of quasi-elastic ν_μ charged-current event reconstruction, including a first evidence of nuclear effects in the region surrounding the interaction vertex.