For several decades, attempts to cryopreserve human oocytes have been performed in many in vitro fertilization (IVF) centers worldwide, with variable results. Ice-free cryopreservation is an attempt to circumvent the hazards of water crystallization as ice. Three key factors influence the probability of successful vitrification: cooling and warming rates, the composition of the cryoprotectants (CPA) solution, and the sample volume. Pressure is another factor that increases the chance of vitrification but this has had very little, if any, application in the clinical assisted reproductive technology (ART) arena. The osmotic stress during removal of CPAs was initially reduced in slow cooling by a stepwise dilution (using reduced concentration of CPAs progressively), allowing enough time for the cell to return to an equilibrium volume. Two of the dangers of cryopreservation are solution effects and intracellular ice formation. Other factors causing damage are extracellular ice and intracellular dehydration.