The dynamic nature of the plastic deformation of metals and its relation to specific thermally activated dislocation mechanisms are discussed. Experimental techniques for evaluating the deformation parameters needed to identify the rate-controlling dislocation mechanism are described. The mechanisms that appear to be controlling in various temperature ranges are listed. Such fundamental lattice properties as the force-distant curve for a thermally activated process, the energy of a dislocation kink or of a jog, the stacking-fault energy, and the activation energy for self diffusion are indicated to be derivable from macroscopic mechanical property data.