Embryonic and fetal growth is an interactive process of intercellular signalling which facilitates early morphogenic events, the condensation and differentiation of organ systems and, in the latter third of gestation, the maturation of those organs necessary for the transition to postnatal life. In the embryo, many of these cellular interactions are direct and involve the deposition and subsequent modification of extracellular matrix, and the temporal expression of cellular recognition molecules. The coordination of the above processes is controlled in individual cells by the expression of transcription factors, and between cells by a phylogenetically ancient and ubiquitous intercellular communication system generically termed peptide growth factors. Growth factors are amongst the first products of the embryonic genome and continue to direct growth and differentiation stimuli until birth and beyond, their actions becoming coordinated with, and in some instances subservient to, a developing endocrine system. In the last few years it has been possible, through molecular biological techniques, to directly test the role of transcription and growth factors in animal models. The use of transgenic and gene function deletion models, mainly in the mouse, has allowed for a better understanding of the molecular and cellular basis underlying many human embryonic and fetal abnormalities and morbidities. A brief introduction will be provided here using growth factors as an example. As a class of messenger molecules, peptide growth factors share a number of characteristics which separate them from classical endocrine hormones. They are widely expressed at both mRNA and peptide levels in developing tissues; they have limited access to target tissues, much of the time due to an ability to bind to extracellular matrix, specific binding proteins, or cell surface molecules other than high affinity receptors; they often rely on proteolytic processing from the target tissue to become bioavailable; and they mostly interact with high affinity cell membrane receptors which signal to the nucleus via a variety of second messenger systems. Liver is a major site of expression of the insulinlike growth factors (IGF-I and -II) in both fetal and postnatal life, although almost all tiss~aes have been shown to express these peptides in the human and animal fetus, suggesting a predominantly autocrine or paracrine role. In the fetus, the most abundant isomer is IGF-II, but in some species such as rat, although not in man, IGF-II is absent from adult serum to be replaced by IGF-I. In humans, IGF-II persists throughout life, although the relative abundance of IGF-I increases postnatally. A high-affinity type-I IGF receptor is ubiquitous in developing tissues, and recognizes IGF-I with a greater binding affinity than it does IGF-II. An additional high affinity receptor which specifically binds IGF-II, the type-II or cation-independent marmoset6+ phosphate receptor, is Ubiquitous but may: be predominantly linked to cell .migration rather thari