More than 25 years ago, low temperature experiments aimed at establishing the ultimate limits to optical storage in solids led to the first optical detection and spectroscopy of a single molecule in the condensed phase. At this unexplored ultimate limit, many surprises occurred where single molecules showed both spontaneous changes (blinking) and light-driven control of emission, properties that were also observed in 1997 at room temperature with single green fluorescent protein variants. In 2006, PALM and subsequent approaches showed that the optical diffraction limit of ∼200 nm can be circumvented with single molecules to achieve super-resolution fluorescence microscopy with relatively nonperturbative visible light. Super-resolution microscopy has opened up a new frontier in which biological structures and behavior can be observed in fixed and live cells with resolutions down to 20-40 nm and below. Current methods development research addresses ways to extract more information from each single molecule such as 3D position and orientation, and both of these can be obtained by proper point-spread function engineering of a wide-field microscope. It is worth noting that in spite of all the current focus on super-resolution, even in the “conventional” low concentration, single-molecule tracking regime where the motions of individual biomolecules are recorded rather than the shapes of extended structures, much can still be learned about biological processes. For example, my laboratory has explored the motions of single Smoothened proteins in the primary cilium, providing evidence of binding sites at the ciliary base whose affinity is modulated by Hedgehog pathway activation. Using 3D precision tracking at high speed, correlations in the motions of pairs of DNA loci in the yeast nucleus highlight the complexity of the nuclear environment through the appearance of subdiffusive motion.