The community of research workers interested in solitons and signal processing came to realise 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. The present book 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, gives the reader a depth of understanding firmly rooted in the basic material.