The concept of affordance is a central component of the ecological psychology of J.J. Gibson. Affordances are properties of the environment taken relative to an observer. Ecological theorists have developed formal models for the analysis of affordances. Models proposed by Shaw and Turvey (1981), Turvey (1992) and Greeno (1994) are described and evaluated and another approach, using Turing’s theory of computation, is outlined. Affordances are characterised as the configurations of Turing machines. It is shown that Turing’s work provides a natural vehicle for exploring Gibson’s ideas.