Forming and forging processes are among the oldest and most important materials related technologies. New materials technologies centre on the development and widespread use of thermomechanical processing, particularly for aerospace alloys and concepts of metal workability and formability. Workability refers to the relative ease with which a metal can be shaped through plastic deformation. The term workability is often used interchangeably with the term formability, which is preferred for the shaping of sheet metal parts. However, workability is usually used to refer to the shaping of materials by such bulk deformation processes as forging, extrusion, and rolling. The characterisation of mechanical behaviour of a material by tension testing measures two different types of mechanical property: strength properties (such as yield strength and ultimate tensile strength) and ductility properties (such as percentage elongation and reduction in area). Similarly, the evaluation of workability involves both the measurement of the resistance to deformation (strength) and determination of the extent of possible plastic deformation before fracture (ductility). Therefore, a complete description of the workability of a material is specified by its flow stress dependence on processing variables (for example strain, strain rate, preheat temperature, and die temperature), its failure behaviour, and the metallurgical transformations that characterise the alloy system to which it belongs. However, the major emphasis in workability is on measurement and prediction of limit of deformation before fracture. Therefore, the emphasis in the present review article is on methods for determining the extent of deformation a metal can withstand before cracking or fracture occurs. 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 behaviour for defining the deformation maps that delineate ‘safe’ and ‘non-safe’ hot working conditions. These maps show in the processing space (that is on axes of temperature T and strain rate ɛ·) the processing conditions for stable and unstable deformation. The range of T and ɛ· for stable material flow useful in the development of process control algorithms should be obtained from physical quantities, which can be evaluated from the test data. As such there is no unique instability theory existing to delineate the regions of unstable flow during hot deformation which is applicable for all the materials. The designer has to establish a suitable theory on the basis of microstructural observations of the flow localisation in the intended materials. Various existing theories are reviewed for identification of flow instabilities (such as adiabatic shear banding, intercrystalline cracking, prior particle boundary cracking in powder compacts, wedge cracking) during hot deformation of materials, and these theories are verified by considering the flow stress data of IN 718 and the reported microstructural observations from different sources.