The study of the molecular adaptation of microorganisms to extreme environments (solvent, temperature, etc.) has provided tools to investigate the complex relationships between protein-solvent and protein-protein interactions, protein stability and protein dynamics, and how they are modulated by the crowded environment of the cell. We have evaluated protein-solvent and protein-protein interactions by solution experiments (analytical ultracentrifugation, small angle neutron and X-ray scattering, density) and crystallography, and protein dynamics by energy resolved neutron scattering. This review concerns work from our laboratory on (i) proteins from extreme halophilic Archaea, and (ii) psychrophile, mesophile, thermophile and hyperthermophile bacterial cells.