Super-resolution optical microscopy, sometimes called optical nanoscopy, refers to a new kind of far-field optical microscopy which allows optical imaging with a resolution higher than the well-known resolution limit (or the Abbe diffraction limit) which stood for more than a century. The reported approaches that break the resolution limit involve either effectively reducing the focus region of the excitation light or stochastic separation and precise localization of single fluorophores. Super-resolution optical microscopy provides unprecedented opportunities for tackling outstanding fundamental questions in life science, medicine, materials, and many others that require non-invasive imaging with molecular level spatial resolution.