Nanospectroscopy addresses the spectroscopy of very small objects down to single molecules or atoms, or high-resolution spectroscopy performed on regions much smaller than the wavelength of light, revealing their local optical, electronic and chemical properties. This work highlights modern examples where optical nanospectroscopy is exploited in photonics, optical sensing, medicine, or state-of-the-art applications in material, chemical and biological sciences. Examples include the use of nanospectroscopy in such varied fields as quantum emitters, dyes and two-dimensional materials, on solar cells, radiation imaging detectors, biosensors and sensors for explosives, in biomolecular and cancer detection, food science, and cultural heritage studies. Also by the editors: Textbook "Optical Nanospectroscopy": _"Fundamentals & Methods" (Vol. 1) and _"Instrumentation, Simulation & Materials" (Vol. 2).