What are severe learning difficulties (SLD) and profound and multiple learning difficulties (PMLD)?

Since being involved in starting up The Bridge Professional Training Centre (part of The Bridge School, an all-age SLD/PMLD/ASD school in Islington) in 1997, I have been exercised by the problem of satisfactorily defining the terms ‘severe learning difficulties’ and ‘profound and multiple learning difficulties’ to those I train. The thoughts outlined in this chapter are a reworking of those first propounded in The SLD Experience (Imray 2005).