In recent years the number of reports describing phenotypically and functionally distinct subsets of human blood leukocytes, and in particular of subtypes of antigen-presenting cells has continuously increased. A great diversity was described not only for dendritic cells (DC), but also for human blood monocytes (Mo) and macrophages (Mac). Similar to DC, the different types of Mo subsets could be defined by distinct phenotypes and immunoregulatory functions. The characterization of blood Mo subpopulations revealed that some of them exhibit common features with myeloid or lymphoid DC and tissue Mac, but also demonstrate the existence of novel unique cell populations. The generation of lymphoid and myeloid DC and their heterogeneity has been the subject of recent reviews. Here we focus on Mo from human peripheral blood and summarize the data (including our own) dealing with their phenotypic and functional, in particular immunoregulatory properties.