Imaging approaches based on single molecule localization break the diffraction barrier of conventional fluorescence microscopy, allowing for bioimaging with nanometer resolution. It remains a challenge, however, to precisely localize photon-limited single molecules in 3D. We have developed a new localization-based imaging technique achieving almost isotropic subdiffraction resolution in 3D. A tilted mirror is used to generate a side view in addition to the front view of activated single emitters, allowing their 3D localization to be precisely determined for superresolution imaging. Because both front and side views are in focus, this method is able to efficiently collect emitted photons. The technique is simple to implement on a commercial fluorescence microscope, and especially suitable for biological samples with photon-limited chromophores such as endogenously expressed photoactivatable fluorescent proteins. Moreover, this method is relatively resistant to optical aberration, as it requires only centroid determination for localization analysis. Here we demonstrate the application of this method to 3D imaging of bacterial protein distribution and neuron dendritic morphology with subdiffraction resolution.3D bioimaging | nanoimaging | bacterial imaging | neuron imaging | biophotonics F luorescence microscopy allows the observation of biological samples in a highly sensitive, selective, and relatively noninvasive fashion. Moreover, fluorescence imaging tools can probe deeply into tissues to obtain volume images without physically isolating different sections. However, traditional fluorescence microscopy suffers from diffraction-limited resolution, far larger than actual molecular sizes. Recent development of two different superresolution (SR) fluorescence imaging methods has broken the diffraction limit and enables direct optical observation of biological features near molecular scales. One approach is to create a subdiffraction excitation pattern by stimulated emission depletion (1, 2) or structured illumination (3, 4), and the other approach is based on single molecule localization (5-9). An essential part of the single molecule localization methods involves centroid fitting of serial single molecule images at the detection plane in order to determine the lateral localization of each fluorescent molecule with precision limited not by diffraction, but mainly by its number of emitted photons. A superresolution 2D image of the sample can then be rendered by overlapping all single molecule localizations from all image frames. This principle of localization has also been applied to single particle tracking (10)(11)(12)(13)(14).Single molecule-based superresolution microscopy requires individual chromophores to be turned "on" and "off" sequentially, either by photoswitching or by other stochastic photophysical processes. Compared to synthetic photoswitchable probes, photoactivatable fluorescent proteins (PA-FPs) (15-20) are superior in many regards: PA-FPs can be genetically fused to target proteins and endogenously...