Results of french multiple collaborative studies are reported to evaluate the possible benefits of coculture in human IVF program. Prospective randomized study of IVF with transfer on day two versus IVF with transfer on day 6 shows that coculture does not improve overall results but leads to a reduction of triple or quadruple pregnancies. When applied to patients with more than 4 failures of embryo transfers on day 2, delayed transfer of blastocyst results in a high rate of clinical pregnancy (39%). Transfers of frozen-thawed blastocysts result in 17.8% ongoing pregnancies. Coculture is also a powerful tool for understanding early embryo development: when fertilization arises from sperm with poor characteristics or from frozen sperm of donor, embryo development appears to be altered: in these situations, rate of normally developing embryo is reduced and early embryo development becomes independent of fertilization rate.