Astrocytes, the most abundant cell type in the brain, proliferate during brain development. While it is generally accepted that mature astrocytes do not proliferate, neural stem cells, which have characteristics of astrocytes, retain the ability of self-renewal. Furthermore, astrocytes can regain their proliferative properties under pathological situations, such as reactive astrogliosis, a consequence of brain injury and brain tumors. Measurements of astrocyte proliferation can thus be used in investigations of physiological and pathological processes occurring in the developing and the adult brain. This chapter describes two methods for the determination of astrocyte proliferation: the incorporation of a radioactive nucleotide [(3)H]thymidine into DNA, which occurs during the process of DNA synthesis preceding cell division, and the flow cytometric analysis of cell cycle progression through the different phases of the cell cycle by BrDu/Hoechst and ethidium bromide labeling.