Aim of this book is to present to researchers and engineers a description of the present knowledge on the viscoplasticity of geomaterials, considering the experimental and theoretical aspects and the applications to relevant problems. First, the laboratory tests performed in order to quantitatively assess the theological properties of rocks are illustrated, describing the equipments used in modern rock mechanics laboratories and the proper testing procedures. Part of the presentation is devoted to the description of constitutive creep laws having different degrees of accuracy and complexity. The book covers also problems related to the application of numerical solution procedures to rock/soil mechanics situations in which the slow deformation of geomaterials is to be taken into account.