Surface-enhanced Raman spectroscopy (SERS) is a spectroscopic technique that simultaneously combines fingerprint recognition capabilities, typical of vibrational spectroscopies, and very high sensitivity (down to single molecule), owing to the enhancement provided by plasmonic effects. Its discovery dates back to the 1970s, and since then, SERS has gained a lot of interest in the scientific community, as witnessed by the quick raise in the percentage of publications involving SERS, especially in the last two decades. In this book chapter, we would like to provide the reader with an overview of SERS, going from the illustration of its basic principles to the description of a wide selection of its applications. At first, the physical phenomena responsible for the electromagnetic and chemical SERS enhancements are described; thereafter, two key features of SERS, namely, its distance dependence and the concept of hot spot, are discussed, as well as the near- vs. far-field properties in plasmonic systems. Two sections are then dedicated to the materials that are more often used in SERS and to the strategies adopted to fabricate efficient SERS substrates. The last section illustrates the applications of SERS in several fields of sensing, like the detection of chemical warfare agents, environmental pollutants, food contaminants, and illicit drugs; the use of SERS in art preservation, forensic science, and medical diagnosis is also described, with specific and relevant examples from the most recent literature.