A consistent requirement for all preimplantation genetic testing is the need to obtain DNA from the oocyte or embryo. Currently this sample is attained through biopsy of one or both polar bodies, blastomere biopsy at the cleavage stage, or trophectoderm biopsy after blastulation. Selecting the optimal time for biopsy requires careful consideration. Polar body biopsy is less invasive and provides more time for analysis but fails to capture as many as one in three embryonic aneuploidies. Additionally, the inability to readily distinguish nondysjunction from premature separation of sister chromatids greatly limits the predictive value of the technique and may lead to an overdiagnosis of aneuploidy in as many as 45% of cases with first polar-body errors. Cleavage-stage biopsy provides adequate samples but is detrimental to the embryo. The adverse effect of blastomere biopsy may result in approximately two of every five reproductively competent embryos losing their ability to implant and sustain development. Trophectoderm biopsy does not adversely impact the embryos. However, for the majority of clinical programs without a genetics laboratory, vitrification would be necessary to allow time for the genetic analysis. Although this extends the time required for treatment, clinical outcomes are equivalent after transfer of euploid blasts during fresh IVF and cryopreserved embryo transfer cycles, so that excellent outcomes are maintained. At present the blastocyst stage is the optimal time to perform biopsies for preimplantation genetic testing.