This review summarizes the current knowledge of human implantation in early embryonic development. Information gained from Carnegie specimens, a limited set of histological specimens that precludes molecular or chronological analysis, is combined with results from the most recent embryo culture platforms. The limitations of existing in vitro cultures are highlighted, particularly the absence of a suitable maternal endometrial environment, and the need for more physiologically relevant systems is outlined. A standardized framework for defining stages and assessing human embryo viability on the basis of morphological, temporal, and functional criteria is proposed.