The femoral nerve is the largest branch of the lumbar plexus, arising from the second, third and fourth lumbar nerves.
It supplies sensation to the anterior thigh and medial aspect of the leg below the knee (via the saphenous nerve) as well as most of the femur and knee joint. It supplies the quadriceps muscle.
To find the nerve (generally for the purpose of nerve blockade) use the ultrasound probe in transverse lie at the level of the femoral crease, centred just lateral to the pulsations of the femoral artery.
At this point the femoral nerve lies lateral to the femoral artery. It is covered by fascia lata, and separated from the femoral artery by fascia iliaca.
Sacha Schweikert with thanks to Graham Hocking and Tor Ercleve