745Creep is a time-dependent process which leads to deformations that are strongly related to a rock’s petrophysical properties. This chapter explains the causes of creep at the microscopic scale. Micro-mechanisms of deformation are described according to the atomic structure of crystals. Relevant laboratory tests, namely creep tests, relaxation tests and strain rate-controlled loading tests are outlined and their performance is discussed. Thereafter, the main rate-dependent constitutive models are listed with comments on their adequacy for different rock types. A summary of the theory of viscoplasticity is provided, along with a procedure for identifying constitutive parameters from test results. In the last section, time-dependent numerical modeling of underground openings is presented with the help of case studies. Particular emphasis is placed on the analysis of time-dependent ground-support interaction.