Recent advances in desk-top computers have resulted in the production of machines which are capable of carrying out most optical design calculations, perhaps with the exception of the optimization of complex multi-configuration systems. We are using the Hewlett-Packard 9845S, and with a 64K byte memory we can optimise systems with up to 40 surfaces, controlling up to 70 aberrations and varying up to 30 parameters. Image assessment can be carried out using spot diagrams, geometrical mtf and diffraction mtf, each of these calculations being carried out within the 64K memory. In interactive programs it is possible for the program to be able to carry out many short calculations, and it is possible to provide graphical output, whenever this is useful. Advantages of desk-top computers for design are economy, convenience and the speed with which results can be obtained, plus the greater understanding that is possible when the designer is in control of the program, and when hard-copy graphics is available. The only disadvantage is the relatively slow computing speed, but this will probably be overcome when new desk-top computers become available. A smaller program, for the H-P 85 will also be described.