Fetal growth is regulated by fetal hormones in all species that have been studied.However, it is clear that strict definition of fetal growth must be applied in order that different studies and different species may be meaningfully compared. The experimental manipulation of fetal growth in vivo has been the main tool by which information in this area has been gained. Organ ablation experiments, with or without appropriate hormone replacement treatments, have been used now for nearly 50 years as a means of studying fetal growth. As the years have gone by, many of the original techniques have been refined beyond recognition, so that precise surgical or immunological approaches have now replaced the rather simpler earlier methods. However, the nature of the questions which are posed are still remarkably similar to those first formulated nearly 50 years ago. What regulates fetal growth? This article attempts to document the progress that has been made in the endocrine control of fetal growth.