This thesis describes the design and implementation of a graphical output system for the KDF9 computer. In particular it describes work undertaken by the author to implement a practical method of using Calcomp 565 drum plotter on-line to the KDF9. The object of the system was to allow a high level language program to make use of the plotter in an efficient manner under the Egdon Operating System. There were two main problems to be overcome, namely to present the user with a usable graphical output language and to overcome the mis match between the speeds of the plotter and the computation, A graphical output language written at U.K.A.E.A, Culham for a Benson-Lehner 120 microfilm printer-plotter and a D.E.C, 338 Display was available and is described in the thesis. This language was accepted as a basis for a graphical output system and it is shown how the language was altered and developed for use on the Calcomp plotter. The Calcomp plotter is on-line to the KDF9 and therefore has to be driven directly from the computer. The plotter is a very slow device compared to the speed of the KDF9. In the Egdon system if plotting were done directly from the user program then the background stream would be held up for long periods of time while plotting was done. This is obviously unsatisfactory and some system of buffering the plotter output had to be devised. This is done by writing the binary commands to drive the plotter on to magnetic tape, batching several jobs on a single tape. The plotting is then carried out pseudo off line. This involves altering the Egdon Supervisor to allow plotting to multiprogram with the rest of the Egdon System. The system as described above achieved all its objectives. Although only a relatively few users have to date made use of the system, it has proved reliable and effective. It also appears to be acceptable for operator use, an important consideration. Finally to provide information for the prospective user, an operating manual 'The Glasgow Calcomp Plotter System' has been written.