This chapter reviews the tremendous progress over the past several decades in experimental research of molecular solids at high pressures. The interatomic interactions in these materials are greatly modified under pressure and generally strengthen intermolecular and weaken intramolecular bonds. This leads to the formation of structurally complex crystals and inclusion compounds at moderate pressures, where a variety of intermolecular bonds can exist. Pressing on, a great majority of molecular solids demonstrate transformations to extended (e.g., polymeric) states, which vary drastically in bonding and electronic properties. The most prominent example of such behavior is the symmetrization of hydrogen bonds in ionic ice X and metallization of hydrogen in monatomic solid. Dave Mao’s legacy in this research has been remarkable ranging from discovering and establishing the structure and properties of hydrogen clathrate hydrates at 200 MPa to investigating the structure of a mixed atomic-molecular phase IV of hydrogen at 260 GPa. New generations of scientists continue to use and build upon his technical developments, which have enabled multimegabar investigations of molecular solids, including diamond anvil cell (DAC) design, the DAC gas-loading system, and a variety of optical, electric, magnetic, and X-ray DAC probes.