Dendritic cells comprise several populations with distinct ontogeny that share core features including a typical dendritic morphology and the ability to present antigens and stimulate T cells. Dendritic cells originating from monocytes have been reported in steady-state and in different inflammatory contexts, in mouse models and in human clinical samples. However, because of their phenotypical and transcriptional proximity with other dendritic cell subsets and with monocyte-derived macrophages, whether monocyte-derived dendritic cells (mo-DCs) represent a distinct population has been controversial. Here, we summarize the evidence supporting the existence of mo-DCs in vivo and we review work addressing the molecular regulation of mo-DCs differentiation and their role in immune responses. We also discuss the potential for harnessing mo-DCs differentiation for therapy.