Recently terahertz spectroscopy becomes popular not only in optical science but also in material science and engineering. Several spectrometers are now commercially available, and anybody has a chance to use this new technique. This review provides four examples of our recent works investigating physical properties of molecular solids using terahertz spectroscopy. The topics are “kinetics of polymorphic transition of cyclohexanol”, “molecular motion of nylon-6 in the crystalline domain”, “structural change in giant cluster of multinuclear metal complex”, and “dynamics of Li⁺ ion encapsulated in C₆₀ fullerene”. The molecular motions observed uncover the structural change, the coupling of motions, and the dynamics of loosely bound atoms or molecules. Temperature dependences of the terahertz spectra reveal the changes in these factors, and it is suggested that terahertz spectroscopy goes well with calorimetry.