In this paper Arntz underlines the need for a sound theoretical base on which LSP research can proceed. This theoretical base can only be attained if we gain insight into special language, taking into account its complexity and various functions, and delineating it from general languages. A second prerequisite would be a description of special languages on all relevant levels, including text level. An important step towards this goal was taken with Sager’s and Dungworth’s pioneering study English Special Languages. Arntz looks at the criteria and principles that must be respected in order to come to a description of LSP. From there he looks at the difficulties of developing special language teaching material for two different target groups; i.e. specialists interested in receptive skills versus advanced translation students.