This brief review is devoted to the nature of early blastomere differentiation in human 4-cell embryos and its consequences for embryonic development. Precursor cells of inner cell mass, germline, and trophectoderm may be formed at this stage, the clearest evidence being available for trophectoderm. The sites of these precursor cells in the embryo could be ascertained using markers for animal and vegetal poles, observing specific cleavage planes, and assessing gene and protein expression. This opens new opportunities for studying 4-cell embryos and removing or replacing specific cells. Knowledge of the properties of individual blastomeres should help in improving assisted human reproduction, performing preimplantation genetic diagnosis, and perhaps establishing specific stem cell lines. Special attention is paid to well-characterized trophectoderm, the trophectoderm stem cell, and possible new forms of clinical application.