The lattice strain relaxation has been experimentally investigated in highly mismatched III-V heterostructures by X-ray diffraction methods. Two kinds of heterostructures have been studied: binary layers on binary substrates (GaAs/Inp, GaAs/GaP, InP/GaAs) and ternary layers on binary substrates (In_xGa_1-x As/GaAs). For the binary-on-binary structures it has been found that the relaxation process is well described by the equilibrium theory, which assumes that a minimum energy state is achieved. However, the critical thickness for initiating the relaxation is usually larger that theoretically predicted. This is due to the fact that in real cases the initial growth usually occurs through island formation and this produces a relaxation smaller that expected in layer-by-layer growth. As for the ternary-on-binary structures, two different behaviours have been observed. For high mismatch values results similar to those obtained in the binary-on-binary samples have been found. On the contrary, for lower mismatches the measured residual strains are much larger than predicted by the equilibrium theory and the relaxation process may be described by assuming that the elastic energy per unit interface area remains constant.