We use reflection electron microscopy (REM) to analyze ion‐etched nanostructures defined in compound semiconductor heterostructures. This nondestructive imaging technique allows us to characterize 10‐nm‐wide features with high resolution and determine their sidewall morphology on a 1 nm scale. In addition to the inherent high‐resolution available from REM, we also obtain diffraction contrast from the heterostructure material, and we can image quantum wells. We routinely use this technique to characterize and accurately measure microfabricated structures with lateral dimensions below 20 nm.