The methods for determining the extent of deformation a metal can withstand before cracking or fracture occurs are reviewed. Flow stress data are essential in the development of constitutive equations and processing maps. One of the requirements for process modelling is a knowledge of the material flow behavior for defining the deformation maps that delineate `safe' and `non-safe' hot working conditions. These maps show in the processing space, the processing conditions for stable and unstable deformation.