Phagocytosis is a cellular process that plays crucial roles in the removal of dead or dying cells, tissue remodeling, and host defense against invading pathogens. Most eukaryotic cells are decorated with glycoproteins containing terminal sialic acids, whose negative charges tend to repel cells, making so-called "nonspecific" phagocytosis a relatively inefficient process. Professional phagocytes are so designated because they express two major classes of receptors on their surfaces that are primarily involved in phagocytosis. Paradoxically, these receptors do not recognize microbes directly, but rather endogenous proteins that become tethered to microbes and target them for destruction. These are the Fcγ receptors that bind to the Fc portion of IgG and the complement receptors (CRs), which bind primarily to cleavage products of the third component of complement, C3. This unit describes assays that are used to measure these two types of macrophage phagocytosis.