Tangling process of dislocations, especially at the beginning, has been continuously observed with a 500 kV electron microscope mainly on aluminum, and relations between cross slip and tangles are clarified. On the basis of these results, the work-hardening processes in metals are discussed. They are classified into the following two types: (A) The obstacle barrier strongly depends on the local deformation history, (B) the barrier is free from the history so that it is always constant. These types are closely related to the stacking fault energy and the homologous temperature; i.e. , deformation accompanied by the loop-formation belongs to the type (A) and deformation through the interaction products of dislocations belongs to the type (B). Various characteristics of deformation, such as necking, the temperature dependence and the effect of solute atoms, can be analysed in terms of these characters of obstacles.