Terahertz (THz) frequency region is located between microwave and infrared regions, and molecular motions and intermolecular interactions in the THz region reflect unique properties of materials, which are important to understand structural stabilization, chemical reactions, and functionalities of molecular systems. We have been applying spectroscopic methods in the THz region to various problems in the research field of molecular science in condensed phases. THz time-domain spectroscopy (TDS) allows us to obtain both the absorption coefficient and refractive index of materials such as molecular crystals, polymers, and liquids. Time-resolved THz spectroscopy has revealed charge carrier dynamics of organic semiconductor materials and nanocrystals. I will make a review on what we are doing using THz waves in the field of molecular science.