Light microscopy has been a revolutionary method for studying biological processes for several reasons. Light is noninvasive, allowing the study of cellular and subcellular dynamics in real time. In addition, the large toolbox of uorescent probes and labeling strategies that are available, allow the detection of specic subcellular components with exquisite molecular specicity and in multiple colors. ese capabilities render light microscopy superior to electron microscopy, which is only suited for imaging xed cells and suers from poor molecular specicity. However, the diraction limit has, until recently, presented an impenetrable barrier to achieving spatial resolution beyond ~200-300 nm in x-y and ~500 nm in z using light. e low spatial resolution has been a major obstacle for observing the make-up of many biological structures (viruses, chromatin, cytoskeleton) and the subcellular organization and distribution of multiprotein complexes, which are smaller than the diraction limit.