The work reported in this thesis is a feasibility study of the possibilities and practical problems of applying speech control and natural language understanding techniques to the use of a computer by a physically disabled person. Solutions are proposed for the overcoming of some of the difficulties and limitations of the available equipment, and guidance given for the application of such systems to real tasks. The use of voice control with a low cost industrial robot is described. The limitations introduced by the speech control hardware, such as restricted vocabulary size and artificial manner of speaking are partially overcome by software extensions to the operating system and the application of natural language understanding techniques. The application of voice control and audio response to common application packages and a programming environment are explored. Tools are developed to aid the construction of natural language understanding systems. These include an extension to the use of an existing context-free parser generator to enable it to handle context-sensitive grammars, and an efficient parallel parser which is able to find all possible parses of a sentence simultaneously. Machine readable dictionary construction is investigated, incorporating the analysis of complex words in terms of their root forms using affix transformations, and the incorporation of semantic information using a variety of techniques, such as semantic fields, the previously mentioned affix transforms, and object-oriented semantic trees. The software developed for the system is written in Borland Pascal on an IBM compatible P C , and is produced in the form of library modules and a toolkit to facilitate its application to any desired task.