The flow stress of a crystal can be decomposed into two main components. The first one reflects the long-range elastic interaction of mobile dislocations with the microstructure (athermic stress) and the second one is the stress necessary to push dislocations over local energy barriers (thermal stress). These local bariers can be different nature: small obstacles, an intrinsic lattice resistance or an umpropitious dislocation core configuration. The dependence of some special prameters (activation area, activation energy, velocity stress exponent)on temperature or on applied stress can decide which of mechanisms is dominant in the course of plastic deformation.