The community of research workers interested in solitons and signal processing came to realize that there is more to the behaviour of crystals displaying second-order nonlinearity than creating second-harmonic sources. It turns out that there are also soliton - or, more correctly, solitary wave - solutions to the coupled equations which involve a balance between the fundamental wave and the co-propagating second-harmonic wave. Second-order nonlinearity, then, is attracting a lot of interest, both in the more traditional phase-matching requirements needed to generate second-harmonic waves very efficiently - see the drive to the blue laser or its alternatives - and in the new area of solitary waves. This volume is devoted to three main themes: solitons, quasi-phase matching, frequency conversion and parametric interactions. The material has been written by distinguished contributors and the range of topics, together with the emphasis put on them, should give the reader a depth of understanding firmly rooted in the basic material.