A new microscopy technique is introduced, image scanning microscopy (ISM), which combines conventional confocal-laser scanning microscopy with fast wide-field CCD detection. The technique allows for doubling the lateral optical resolution in fluorescence imaging. The physical principle behind ISM is similar to structured illumination microscopy, by combining the resolving power of confocal-laser scanning microscopy with that of a wide-field imaging microscopy. This Letter describes the theoretical foundation and experimental realization of ISM.