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Infrared spectroscopy is discussed as an analytical technique used to detect bond vibrational frequencies. Characteristics of electromagnetic radiation are described including the electromagnetic spectrum, infrared (IR), visible, and ultraviolet (UV) radiation, wave properties, energy considerations, and interpretation of color. Advantages and limitations of dispersive and Fourier transform spectrophotometers are discussed and compared using instrument design and operation, that is, radiation sources, monochromators (dispersive), interferometers (FTIR), and detectors. IR active bond vibrational modes commonly found in drugs of forensic interest are illustrated and categorized. IR spectra interpretation includes a complete discussion on wave numbers (cm−1) versus wavelength, and absorbance versus % transmittance. In addition, sample preparation techniques are described for liquid/vapor and solid sample FTIR including Nujol mull, cast film A and B, KBr pellets, and synthetic membrane sample cards.