Foelling, J., Bossi, M., Hannes, B., Medda, R., Wurm, C. A., Hein, B., Jakobs, S., Eggeling, C. and Hell, S. W.
Fluorescence nanoscopy by ground-state depletion and single-molecule return. Nature Methods 5: 943-945 (2008).

We introduce far-field fluorescence nanoscopy with ordinary fluorophores based on switching the majority of them to a metastable dark state, such as the triplet, and calculating the position of those left or those that spontaneously returned to the ground state. Continuous widefield illumination by a single laser and a continuously operating camera yielded dual-color images of rhodamine- and fluorescent protein–labeled (living) samples, proving a simple yet powerful super-resolution approach.