When a sample is irradiated with light, the light is scattered at a different wavelength from the incident light due to the inherent molecular vibration of the molecules in the sample. This effect is called Raman scattering (C. V. Raman received the 1930 Nobel Prize in physics for his discovery of the effect). The scattered light is detected and measured to identify the molecules in the sample. This analytical method is called Raman spectroscopy. Compared to other analytical methods such as fluorescence detection, X-ray analysis, NMR analysis, and mass spectrometry, Raman spectroscopy is very simple and does not require prior knowledge of the composition of the sample, but it suffers from low sensitivity because only one out of a million incident photons is Raman-scattered. SERS solves this low-sensitivity problem by placing the sample on a metal nanostructure, where the interaction of the incident light with the metal nanostructure causes an electromagnetic-field enhancement due to an effect called localized surface plasmon resonance, which greatly enhances the Raman-scattered light by several orders of magnitude or higher and is used to identify molecules in the sample with high sensitivity.