The appearance and phenotype of histiocytes can change with levels of maturation, activation, and the local tissue microenvironment, with only a few markers truly constitutive. Widely varying surface and cytoplasmic molecules are informative for these cells and are easily identified using flow cytometry, although many are not unique to these cell lines. Others are more useful for the identification of macrophages, dendritic cells, and their neoplastic counterparts in fixed tissues. Table 25.1 lists some antibodies informative for histiocytes by flow cytometry, only some of which are restricted to either macrophages or dendritic cells; many are common to both. Table 25.2 lists the common types of monocyte/macrophage and dendritic cells and the markers most useful in their identification in fixed tissues (1). Chapter 28 further highlights immunostain panels best adapted for neoplastic proliferations.