Mastering of the terahertz (THz) spectral range has motivated intensive studies aimed at developing applications of THz radiation. Spectroscopy is one of them, as molecular absorption lines corresponding to the vibrational and/or rotational frequencies belong to the THz range. Subwavelength sample thicknesses impose great difficulties to conventional THz spectroscopy, yet sensing of such samples is essential. Potentialities of thin film THz spectroscopy are considerably enlarged if a sample is probed by surface plasmon-polaritons (SPP) - the complex of an evanescent EM wave coupled to a wave of free charges propagating nonradiatively along the surface. The book describes principles and achievements in the field of THz SPP spectroscopy. Two concepts (absorption and amplitude-phase) of the method are described. Designs of static and dynamic SPP spectrometers, techniques for determining the dielectric function of metals via measured characteristics of SPP, interferometers able of measuring the SPP complex refractive index and principles of dispersive THz SPP Fourier-transform spectroscopy are considered.