This comprehensive two-volume graduate textbook presents the theoretical basis of optics and spectroscopy together with their advancement to the nanoscale, relevant experimental and numerical techniques, and selected emerging applications of nanospectroscopy. The work aims at students of the natural sciences and interested readers entering this fascinating interdisciplinary field. Volume 1 covers the basics of optical nanospectroscopy from the fundamentals of light-matter-interaction, through an overview over optical spectroscopies in general, to the specific techniques in nanospectroscopy that either offer nanoscale resolution or are addressed to nano-objects. Vol. 2: Instrumentation, Simulation & Materials. The work "Optical Nanospectroscopy. Applications" presents numerous examples in modern photonics and optical sensing or state-of-the-art applications in material, chemical and biological sciences.