The advantages of using very high voltages for electron microscopy are reviewed, in respect of greater specimen thickness and reduced chromatic aberration. For polymers and possibly with living material, the lower radiation damage in a given thickness is also valuable. The special problems in the design and construction of microscopes operating up to 1 Mv are discussed. We are still in a stage of rapid development, and some possibilities for the future are outlined. The main areas of application of high voltage microscopes are beginning to be defined; some examples are given of what has been done with the Toulouse and Cambridge microscopes to date.