▪ Abstract Cryo-electron microscopy (cryo-EM) of biological molecules in single-particle (i.e., unordered, nonaggregated) form is a new approach to the study of molecular assemblies, which are often too large and flexible to be amenable to X-ray crystallography. New insights into biological function on the molecular level are expected from cryo-EM applied to the study of such complexes “trapped” at different stages of their conformational changes and dynamical interactions. Important molecular machines involved in the fundamental processes of transcription, mRNA splicing, and translation are examples for successful applications of the new technique, combined with structural knowledge gained by conventional techniques of structure determination, such as X-ray crystallography and NMR.