Almost every disorder of the CNS is said to have an inflammatory component, but the precise nature of inflammation in the CNS is often imprecisely defined, and the role of CNS-resident cells is uncertain compared with that of cells that invade the tissue from the systemic immune compartment. To understand inflammation in the CNS, the term must be better defined, and the response of tissue to disturbances in homoeostasis (eg, neurodegenerative processes) should be distinguished from disorders in which aberrant immune responses lead to CNS dysfunction and tissue destruction (eg, autoimmunity). Whether the inflammatory tissue response to injury is reparative or degenerative seems to be dependent on context and timing, as are the windows of opportunity for therapeutic intervention in inflammatory CNS diseases. Copyright © 2015 Elsevier Ltd. All rights reserved.