This chapter provides a general description of near-infrared (NIR) spectroscopy and its relevance as an analytical tool. It is addressed at people who do not have a deep knowledge of vibrational spectroscopy but want to learn about the analytical potential of this technique, its benefits, and its limitations. The chapter demonstrates that NIR can be applied to address a wide range of issues and provide solutions for rapid analysis in the laboratory and in the industry. The harmonic oscillator provides an understanding of the concept of vibrational energy, but cannot be applied to molecules because molecular systems cannot assume the continuous energy profile described by the classical model. Some of the publications focus on the determination by NIR spectroscopy of the major constituents in foods: moisture, protein, fat, and carbohydrates. NIR spectroscopy allows all types of samples to be studied so long as a suitable sampling technique is designated.