A physicist talking on mechanical properties of materials will obviously start from the properties which are simplest from the point of view of a mathematical physical description. These are the properties exhibited by the material at small deformations, so small that linear relations exist between the deformation and the stress. This linear behaviour of materials can be described completely by two functions of time. For larger deformations and particularly for the behaviour at rupture a complete physical description has not yet been obtained and the difficulties involved in reaching such a description are tremendous. However, the engineer is most particularly interested in those latter properties, and although the information obtainable from physical measurements in this field is very incomplete, additional information and insight can be acquired with the methods of the biologist, i. e. by visual examination with an appropriate microscope.