Life grows almost everywhere on earth, including in extreme environments and under harsh conditions. Organisms adapted to high temperatures are called thermophiles (growth temperature 45-75 degrees C) and hyperthermophiles (growth temperature >or= 80 degrees C). Proteins from such organisms usually show extreme thermal stability, despite having folded structures very similar to their mesostable counterparts. Here, we summarize the current data on thermodynamic and kinetic folding/unfolding behaviors of proteins from hyperthermophilic microorganisms. In contrast to thermostable proteins, rather few (i.e. less than 20) hyperthermostable proteins have been thoroughly characterized in terms of their in vitro folding processes and their thermodynamic stability profiles. Examples that will be discussed include co-chaperonin proteins, iron-sulfur-cluster proteins, and DNA-binding proteins from hyperthermophilic bacteria (i.e. Aquifex and Theromotoga) and archea (e.g. Pyrococcus, Thermococcus, Methanothermus and Sulfolobus). Despite the small set of studied systems, it is clear that super-slow protein unfolding is a dominant strategy to allow these proteins to function at extreme temperatures.