Mammalian spermatozoa first acquire the ability to fertilize oocytes as they pass through the epididymis to mature. Due to recent advances in microinsemination techniques, not only mature spermatozoa, but also immature sperm cells at certain stages in the testis, have been used to construct diploid zygotes, some of which subsequently develop to normal offspring. Using round spermatids, the most youngest haploid male germ cells, normal births have been reported in the mouse, rabbit, and human. Furthermore, in the mouse, secondary and primary spermatocytes also support full term development after incorporation into immature or mature homologous oocytes. Spermatogenic cells of several species can be cryopreserved easily in simple cryoprotectant solutions. Thus, the microinsemination techniques using spermatogenic cells give us a way to treat infertility, and provide valuable information on gametogenesis, including spermatogenesis, meiosis, and genomic imprinting.