The fluorescence microscope, especially its confocal variant, has become a standard tool in cell biology research for delivering 3D-images of intact cells. However, the resolution of any standard optical microscope is at least 3 times poorer along the axis of the lens that in its focal plane. Here, we review principles and applications of an emerging family of fluorescence microscopes, such as 4Pi microscopes, which improve axial resolution by a factor of seven by employing two opposing lenses. Noninvasive axial sections of 80-160 nm thickness deliver more faithful 3D-images of subcellular features, providing a new opportunity to significantly enhance our understanding of cellular structure and function.
