**8. Conclusions and future directions**

Vitrification is a very promising cryopreservation method with many advantages, and an ever increasing clinical track record. A standardized vitrification protocol applicable to all stages of the pre-implantation embryo may not be realistic because of: **a)** different surfaceto-volume ratios; **b)** differing cooling rate requirements between oocytes, zygotes, cleavage stage embryos and blastocysts; and **c)** variable chill-sensitivity between these different developmental stages. Currently however, the most widely used protocol applied to any embryo stage is the two-step equilibration in an equi-molar combination of the cryoprotectants ethylene glycol and DMSO, at a concentration of 15% each (v/v) supplemented with 0.5 mol/l sucrose.

For the adoption of vitrification in ART, as with all new technologies, there has been initial resistance; but as clinical data has been accrued, this technology is becoming more commonly adopted as standard procedure in many IVF programs worldwide. With this increased use in human assisted reproduction will come evolution of the vitrification process as it is fine tuned to clinical needs, so pushing forward its development to higher levels of clinical efficiency, utilization and universal acceptance.
