Macrophages are the heterogeneous grouping of cells that are derived from monocytes. They have a multitude of functions depending on their final differentiated state. These functions range from phagocytosis to antigen presentation to bone destruction, to name a few. Their importance in both the innate and acquired immune functions is undeniable. Xenobiotics that degrade their functional status can have grave consequences. In this chapter, we provide an overview of the types of macrophages, their hematopoietic origin and a general discussion of the many different assays that are used to assess their functional status.