Received August 29, 2007
Accepted September 02, 2007

The selection of the embryo with the highest chances of implantation and pregnancy is a key point for the success of human assisted reproduction technologies, mainly due to the worldwide drift to single embryo transfer. Several methods are being proposed without reaching a general consensus on which one, on its own, would be the one with the strongest predictive power for implantation and gestation. Qualitative methods evaluate morphological characteristics from the zygote to the blastocyst stage. On the other hand, indirect physiological methods of embryo viability, such as pronuclear and cleavage dynamics and the beginning of compaction are receiving each time more attention and credibility. None of these methods can, however, identify the embryos with normal chromosomal complement, being that up to 50% or more of the embryos considered suitable for transfer are chromosomally abnormal. Certainly development and improvements of sophisticated methods to directly analyse embryonic metabolism will represent the best tool for embryo selection.