Elasticity imaging is a rather recent non-invasive imaging modality which provides in-vivo data about the viscoelastic properties of tissue. With manual palpation being an integral part of many diagnostic procedures, it is obvious that elasticity imaging has many interesting and promising potentials in medical imaging, i.e. from tissue/lesion characterization over therapy follow-up to guidance during interventions which involve ablation. The general concept of this method is to displace the material mechanically and infer from displacement measurements the intrinsic local viscoelastic properties. Many different technical realizations exist (static, dynamic, transient) utilizing different imaging modalities (MRI, ultrasound) which all probe different frequency domains. Since viscoelastic properties of tissue change strongly with frequency, care must be taken when interpreting the data in terms of elastic and viscous component. In this review we will focus on the dynamic 3D approach via MRI, i.e. a mono-frequent mechanical excitation and a volumetric assessment of the displacement field. This allows overcoming several physical difficulties: firstly compressional waves can be properly suppressed via the application of the curl-operator, secondly waveguide effects are eliminated and finally the calculation of the complex shear modulus does not necessitate any assumption of the underlying rheological model. Clinical results on a large patient collective show that mechanical parameters are very pertinent for the differentiation between low-grade and mid/high-grade liver fibrosis. They outperform the well establish classical APRI blood test as well as a 1D ultrasound-based approach for elasticity imaging.