This book is about the application of Continuos Damage Mechanics (CMD) on both the numerical and experimental study of ductile damage and viscosity of steel at the high temperature tipical of lamination (1300 °C). The numerical approach is realized by the implementation of the Lemaitre damage model as a UMAT subroutine in ABAQUS. The working list of the code is provided in annex. The experimental technique is based on ultrasonic measurements of carefully prepared samples in order to measure the elastic module degradation induced by the damage as a decrease of the sound speed in the material. The samples were further investigated using a tomographic approach and the two methodologies were compared. Here is the outline of the dissertation: 1-A gentle introduction to Damage and Plasticity in Metals. 2-Ductility and Damage at Room Temperature. 3-Porosity and Damage Measurement in Ductile Metals. 4-Single Equation Lemaitre Damage model. 5-Viscosity Damage Model. 6-UMAT Subroutine Code.