This review presents a broad overview of chorionic glycoproteins encoded by the Pregnancy-Associated Glycoprotein (PAG) gene family and also serves to illustrate how the recent discovery of the PAG family has contributed to our general knowledge of genome evolution, placental transcription and placental protein expression. The complex and large PAG family is restricted to the Artiodactyla order, although single PAG-like genes have also been identified in species outside the Artiodactyla. The PAGs are members of the aspartic proteinase (AP) superfamily. Unexpectedly, however, some members of the PAG family possess amino acid substitutions within and around the active site that likely render them unable to act as proteinases. This paper summarises the available information regarding biodiversity of PAG gene expression based on cDNA cloning, mRNA localisation studies and the structural organisation of the PAG genes with a particular emphasis on PAG promoters. It also compares available data regarding PAG protein purifications, sequencing and their N-glycodiversity. Finally, it discusses the scientific relevance, possible functional roles of the PAGs and describes possible profitable applications related to the detection of PAG proteins in the blood of pregnant domestic and wild species.