High-resolution electron microscopy (HREM) has proved invaluable in the study of local structure-i.e. defects-which in many cases govern the physical, chemical, and electrical properties of materials. The use of HREM in investigations of cuprate high-temperature superconductors is reviewed, especially for compounds in which substituents replace some of the copper. HREM can also be very successfully combined with other techniques, such as electron diffraction, as is demonstrated by results of research on the structure and phase transitions of C60 and C70.