The European Society of Human Reproduction and Embryology (ESHRE) Preimplantation Genetic Diagnosis (PGD) Consortium data collection shows that the most widely used biopsy procedure is indeed cleavage-stage biopsy. Cleavage-stage biopsy of human preimplantation embryos always involves two steps: opening of the zona pellucida (ZP) and subsequent removal of cellular material. The opening of the ZP by laser technology has been introduced in clinical embryo biopsy practice more recently. Compaction in the mammalian preimplantation embryo is an essential event that leads to the formation of the trophectoderm, the inner cell mass and the blastocele. Embryos for PGD by means of PCR are ideally obtained by micro-injection of a single sperm cell in order to avoid contamination with naked sperm DNA. Embryo post- biopsy development can be evaluated on a day, where doubling of cells and/or signs of compaction represents good evolution.