The myeloid cell lineage is a fundamental element of the immune system and it can give rise to a diverse set of terminally differentiated cells, such as macrophages or osteoclasts among many others. Mutations or misregulation of gene expression may lead to severe clinical conditions, such as arthritis, osteoporosis or cancers. Epigenetics, the regulation of gene expression and chromatin remodelling, is implicated in cell differentiation, function and disease, and hence it is a promising new area to explore in order to explain underlying cellular mechanisms. Firstly, human macrophage subtypes were studied. Chemokine (C-C motif) ligand (CCL) 1 and mannose receptor were validated to be granulocyte macrophage (GM) colony stimulating factor (CSF) induced macrophage markers, while CCL₂ was specifically expressed in macrophage CSF (MCSF) macrophage population. By utilising publicly available high-throughput sequencing data, new biomarkers dehydrogenase/reductase (SDR family) member 2 and CCL₂₆ were discovered to be MCSF-macrophage specific while guanylate binding protein 5 and apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3A were highly up-regulated in GMCSF cells. Secondly, a range of gene knock-down techniques for the myeloid cell lineage were optimised and established. Lentiviral short-hairpin RNA (shRNA) delivery methods were shown to induce an undesirable pro-inflammatory response in macrophages. Furthermore, the frequently utilised cytomegalovirus promoter for gene expression was shown to be completely silenced in macrophage populations. Locked nucleic acids were selected as a suitable alternative to shRNA knock-down and by employing this new tool it was shown that a histone demethylase lysine (K)-specific demethylase (KDM) 6B is fundamental for macrophage differentiation. Finally, a small molecule GSK-J₄, a potent inhibitor of histone demethylases KDM6A, KDM6B and KDM₅B specific for H₃K_27me3 and H₃K_4me3, respectively, was used to dissect epigenetic signalling in osteoclasts and multiple myeloma. In osteoclasts it was shown to act mainly by inhibiting transcriptional changes required for osteoclastogenesis when MCSF-macrophages are stimulated with Receptor Activator Of Nuclear Factor Kappa-B Ligand (RANKL), as indicated by the differential increase in H₃K_27me3 marks, leading to inhibition of c-Jun and potentially abolition of transcription factor AP-1, required for the transcriptional initiation of nuclear factor of activated T-cells 1 (NFATc1). In multiple myeloma cells, GSK-J₄ causes a dramatic increase in expression, further supported by the build-up of global H₃K_4me3 marks, which results in the upregulation of the unfolded protein response pathway. In both cell systems, there is an early upregulation of metallothionein genes, which in multiple myeloma was shown to increase potentially due to rapid influx of zinc ions within the first 30 minutes, and as such may cause induction of apoptosis in multiple myeloma and may inhibit differentiation of osteoclasts.