Vitrification is an alternative approach to cryopreservation that enables hydrated living cells to be cooled to cryogenic temperatures in the absence of ice. Vitrification simplifies and frequently improves cryopreservation because it eliminates mechanical injury from ice, eliminates the need to find optimal cooling and warming rates, eliminates the importance of differing optimal cooling and warming rates for cells in mixed cell type populations, eliminates the need to find a frequently imperfect compromise between solution effects injury and intracellular ice formation, and enables cooling to be rapid enough to "outrun" chilling injury, but it complicates the osmotic effects of adding and removing cryoprotective agents and introduces a greater risk of cryoprotectant toxicity during the addition and removal of cryoprotectants. Fortunately, a large number of remedies for the latter problem have been discovered over the past 30+ years, and the former problem can in most cases be eliminated or adequately controlled by careful attention to technique. Vitrification is therefore beginning to realize its potential for enabling the superior and convenient cryopreservation of most types of biological systems (including molecules, cells, tissues, organs, and even some whole organisms), and vitrification is even beginning to be recognized as a successful strategy of nature for surviving harsh environmental conditions. However, many investigators who employ vitrification or what they incorrectly imagine to be vitrification have only a rudimentary understanding of the basic principles of this relatively new and emerging approach to cryopreservation, and this often limits the practical results that can be achieved. A better understanding may therefore help to improve present results while pointing the way to new strategies that may be yet more successful in the future. To assist this understanding, this chapter describes the basic principles of vitrification and indicates the broad potential biological relevance of vitrification.