Microglia are the principal immune effector cells of the central nervous system (CNS). Under normal conditions, they occupy a quiescent surveillance phenotype, but following stimulation by microorganisms or inflammatory cytokines, microglia transform into highly activated migratory, phagocytic cells producing inflammatory cytokines and chemokines. Significantly, several studies have demonstrated that astrocytes attenuate microglial activation, reducing microglial adhesion, production of interleukin-12 (IL-12) and reactive oxygen species (ROS), and expression of inducible nitric oxide synthase (iNOS). In this chapter, we describe an astrocyte-microglia coculture system that can be used to investigate interactions between these two cell types. We also describe a flow cytometry approach to quantify microglial activation state, as assessed by microglial expression of cellular activation markers, including MHC class I and the Mac-1 and α4 integrins.