Pressure-induced structural change in molecular systems has demonstrated strong promises to access previously unexplored, novel structures and new properties in molecular materials with practical applications. The in situ structural characterization is of fundamental importance to understand the exotic structures and the possible transformation mechanisms. Among all the spectroscopic probes, vibrational spectroscopy that include Raman and Fourier-transform infrared (FTIR) spectroscopy and microscopy allow for highly efficient, sensitive and qualitative characterization of pressure-induced new structures and transformation processes in situ. Supported by state-of-the-art, highly customized spectroscopic systems in-house and at synchrotron facilities, molecular structures and materials properties can be probed in a broad pressure-temperature range with very high spectral and spatial resolutions. Complementary to each other, Raman and IR spectroscopy provide valuable information in molecular structures, nature of bonding, lattice dynamics as well as intermolecular interactions. In this chapter, a comprehensive and critical review of examples of pressure-induced molecular transformations in a wide variety of molecules and materials probed by vibrational spectroscopy is provided. The purpose of this chapter is to give readers the most recent advances in high-pressure chemistry and materials research by demonstrating the power of vibrational spectroscopy as a highly effective in situ structural characterization tool.