The resolving power of the electron microscope as assessed by purely electron optical factors is of the order of 1 A. The resolution obtainable in practice is limited by adventitious instabilities, mechanical and electrical in nature. The detailed design of a high resolution microscope follows from an analysis of these disturbances; its construction must be carried out with the highest precision. Special attention is paid to the electron gun, to the specimen stage and to the mounting of the microscope. For the Cambridge project, 600 kV has been adopted on the grounds of cost-effectiveness. It employs a lanthanum boride cathode and high stability electronics. A pneumatic suspension system supports the microscope when in operation, to isolate it from ambient vibrations. From the electron optical parameters of the condenser-objective lens, together with the recorded levels of residual disturbances, an image resolution of 2.0 A is predicted (at 600 kV), which should be improved to 1.5 A by image processing. Initial results from thin specimens of minerals, metal particles and metallic glasses demonstrate that this performance is already closely approached.