Second-harmonic generation (SHG) can be produced by phase matching using the birefringence of nonlinear crystals via the modal dispersion in the case of optical waveguides. Such an approach limits the range of frequencies which can be doubled and also the choice of the nonlinear coefficients. One solution to both problems is to modify the crystal so as to have regions of periodic domain polarity. Whilst this approach does not allow a perfect phase match between the fundamental and harmonic, it nevertheless can be entirely constructive throughout the interaction length of the material and is termed quasi-phase matching (QPM). Periodic modulation of the nonlinear coefficient along the direction of propagation can achieve conversion efficiencies up to 20 times greater than with previous methods. Candidates of interest for quasi-phase-matching are wide band gap inorganic crystals such as LiNbO₃, LiTaO₃ and KTP, and also organic materials if they are transparent, stable against optical damage and have large nonlinear coefficients. To achieve QPM a variety of methods are being tried in order to invert domains periodically, either during the crystal growth phase, or subsequently by altering the lattice of the crystal. For inorganic ferroelectrics most effort has been concentrated on domain inversion in LiNbO₃ and LiTaO₃. Techniques have included application of pulsed electric fields, fields generated during electron bombardment, thermal pulsing or chemically driven movement of lithium. Many of the methods are semi-empirical in that the mechanisms by which the lattice re-structures are poorly understood. This review will therefore not only list the methods that are currently being used, but also comment on the underlying physical processes which allow, or prevent, the re-structuring of the lattice and the domain walls, whilst preserving the non-centrosymmetric characteristics of the lattice. An understanding of mechanisms is valuable for related poling applications in other crystals and it is further noted that many amorphous systems, including glasses used for optical fibre communication, may be stimulated to show periodic structural changes although the usage precedes the knowledge of the mechanisms. The commercial applications and research possibilities for efficient SHG guarantee that this topic area will continue to be central to photonics for a considerable time.