The placenta is the first organ the fetus makes and is the interface between the mother and baby. A normal functioning placenta is crucial for a successful pregnancy. As the placenta develops, highly branched villous tree-like structures form which contain fibroblasts, immature capillaries and macrophages, termed Hofbauer cells (HBC). HBC are the only fetal immune cell population found in the healthy placenta throughout pregnancy. The roles of HBC at different stages of pregnancy are unclear, but they are likely to be important in placental development. We sought to provide an in-depth, high-dimensional characterisation of HBC within first trimester and full-term placenta, understand how changes in their function aid placental physiology, and investigate potential factors driving HBC characteristics. Whole-mount immunofluorescence imaging revealed that HBC morphology within placental villi changes during gestation; appearing as rounded cells in the first trimester compared to highly elongated and branched cells at full term. Live ex vivo imaging demonstrated that HBC were motile in the first-trimester placenta and sessile by full term. A high-dimensional mass cytometry (CyTOF) panel was developed and used to show that within the first trimester placenta HBC were a homogeneous population, lacking expression of the HLA class II marker HLA-DR. This contrasted with full-term placenta where HBC displayed heterogeneous HLA- DR expression. Analysis of publicly available 10X Genomics single-cell RNA-sequencing data determined that full-term HBC heterogeneity was restricted to variable HLA-DR expression. Furthermore, a robust bulk RNA-sequencing dataset was generated to profile placental macrophages from the first trimester and full-term placenta. This allowed for in-depth transcriptomic analyses which determined exceptional similarity between full-term HBC subsets. Functional enrichment analyses revealed that first trimester HBC were associated with placental processes such as hormone synthesis, trophoblast migration and vasculogenesis. Contrastingly, full-term HBC were associated with complement regulation, angiogenesis, and branching morphogenesis. An optimised in vitro culture system was established to show that the HLA-DR expression profile of HBC remains unchanged upon culture and HBC are resistant to IFNγ-inducible HLA class II expression. ATAC-seq revealed that chromatin accessibility around HLA class II genes and the master regulator transcription factor CIITA was reduced in first trimester HBC compared to full-term HBC and monocytes. Lastly, monocyte-derived macrophage cultures demonstrated the importance of M-CSF for driving key features of the HBC phenotype. In summary, the results presented in this thesis provide an in-depth characterisation of HBC and how they change across gestation to aid placental function in a healthy pregnancy.