Near-infrared (NIR) spectroscopy is a technique with broad and varied applications in pharmaceutical analysis. The NIR spectral region lies between the ultraviolet (UV)-visible and infrared regions, and at one time was considered the forgotten region of the electromagnetic spectrum. It is a branch of vibrational spectroscopy that shares many of the principles that apply to other spectroscopic techniques. The NIR spectral region comprises two sub-ranges (see Figure 1) associated with detectors used in instrumentation. The short-wavelength (Herschel) region extends from approximately 780–1100 nm (12,821–9000 cm⁻¹), and the longer wavelengths, which fall between 1100 and 2500 nm (9000–4000 cm⁻¹), compose the traditional NIR spectral region. It is common to express the wavelength (λ) in nanometers (nm) and the frequency (v) in reciprocal centimeters (wavenumber) as acquired by the instrument. Usually, Fourier-transform (FT) spectrometers report the x-axis in wavenumber (cm⁻¹), whereas a dispersive, monochromator-based instrument will show the x-axis in wavelength (nm).