There have been many recent developments in elaborating the approaches for stabilizing enzymes by stabilizing the folding state, destabilizing the unfolded state and altering the kinetics of unfolding. However, these represent a series of rules of thumb rather than the reliable principles that would be expected of 'engineering'. Stability is taken to include thermodynamic stability as measured by reversible denaturation, and kinetic stability as measured by the unfolding rate for enzymes that are subject to irreversible denaturation. As such, the factors that affect stability of the folded state versus the unfolded state, as well as factors affecting rates of folding and unfolding, all play a role in maintaining stability. Recent studies on the role of kinetics and the effect that site-specific substitutions have on transition-state free energies add to our understanding of the factors that determine whether or not a particular substitution will result in the measurable stabilization of a protein.