The knowledge of brain tissue mechanical properties is a critical issue in the head injury biomechanics research field. Due to experimental limitations, brain tissue has been characterized in vitro, whereas its actual in vivo behavior remains poorly known. We propose the use of Magnetic Resonance Elastography as a non invasive, non destructive tool for measuring the viscoelastic properties of brain tissue. The general method and its validation versus rotational rheometry are described, and preliminary results obtained in vivo are presented.