The concept of a generalised, integrated treatment of linear models may also be applied to the analysis of categorical data. During the last thirty years major changes have taken place in the procedures available for analysing categorical data in the social and environmental sciences. In the 1950s, most of the procedures available tended to be rudimentary and ad hoc, a collection of techniques which was difficult to apply or adapt to unusual types of problem. Since then, a number of developments have occurred which have revolutionalised the situation.