The terahertz (THz) region offers a multitude of possibilities for new diagnostic techniques in biology. Advancements in terahertz sources and detectors have allowed such systems to be employed in investigations of biological tissues. Despite the limited science and technology compared to the neighboring spectral bands, there has been an increasing interest in THz radiation due to the many interesting biological phenomena that can be investigated in this frequency range—typically accepted to be from 0.1 to 10 THz. Plant and animal tissues were investigated using a system similar to the THz-time-domain spectroscopy (TDS) method. In THz-TDS, the electric field of the THz pulse is measured. Since the electric field has both amplitude and phase information, the absorption coefficient and refractive index of the sample can be obtained by Fourier transform of the electric field. In the mid-and far-infrared regions, THz-TDS and Fourier transform infrared are two complimentary spectroscopic methods.