Correlated light and electron microscopy: ultrastructure lights up

Correlated light and electron microscopy (CLEM) gives context to biomolecules studied with fluorescence microscopy. This Review discusses recent improvements and guides readers on probes, instrumentation and sample preparation to implement CLEM. Microscopy has gone hand in hand with the study of living systems since van Leeuwenhoek observed living microorganisms and cells in 1674 using his light microscope. A spectrum of dyes and probes now enable the localization of molecules of interest within living cells by fluorescence microscopy. With electron microscopy (EM), cellular ultrastructure has been revealed. Bridging these two modalities, correlated light microscopy and EM (CLEM) opens new avenues. Studies of protein dynamics with fluorescent proteins (FPs), which leave the investigator 'in the dark' concerning cellular context, can be followed by EM examination. Rare events can be preselected at the light microscopy level before EM analysis. Ongoing development—including of dedicated probes, integrated microscopes, large-scale and three-dimensional EM and super-resolution fluorescence microscopy—now paves the way for broad CLEM implementation in biology.