The culture and critical assessment of early human embryos during the first week of human development are reviewed for the derivation of ESC. Both normal and abnormal features are assessed by phase contrast microscopy of whole embryos and in serial sections of fixed material by light and electron microscopy (TEM). Normal embryos follow a time table of development and have equal blastomeres with minimal fragmentation and nuclear defects. Abnormal embryos show more fragmentation and nuclear aberrations such as micronucleation and multinucleation, reflected by aneuploidy, polyploidy, and mosaicism. The selection of normal embryos and the hardiest of embryos that survive to blastocysts is recommended for the derivation and culture of ESC.