50 years after the first basic idea of Peierls, a review is made on the different aspects of friction forces in metals and alloys. Different approachs are compared, and it is shown that all lead to similar results, corresponding to different behaviours in two stress ranges. Experimental results in BCC, HCP, and FCC metals, are in good agreement with these theoretical developments. In a second part, an extension of the Peierls model, called locking-unlocking, is developed on the basis of experimental results in the prismatic planes of beryllium. It applies to many other situations, such as prismatic glide in magnesium and titanium, cube and octahedral glide in LI2 nickel-based ordered alloys, and possibly some BCC metals. The relation between locking-unlocking and Peierls mechanisms is discussed, as the role of locking-unlocking in the formation of strength anomalies.