Dr. Peter J. Hansen has conducted basic and applied research to formulate new concepts regarding the nature of the interaction between the maternal system and developing embryo. The new understanding generated by his work has been used to develop innovative approaches to improving female fertility and increase resistance of animals to heat stress. His work, detailed in over 330 peer-reviewed publications, has resulted in use of embryo transfer to alleviate maternal infertility caused by heat stress. He has characterized genes that confer thermotolerance in dairy cattle and developed cattle that are genetically resistant to heat stress. His research has pointed the way for development of methods to alter postnatal phenotype through modification of preimplantation development. Studies on colony stimulating factor 2 and the methyl donor choline have revealed how molecules in the reproductive tract can regulate embryonic development to modify postnatal phenotype of the resultant offspring.