Interleukin-33 (IL-33), a newly recognized IL-1 family member, is expressed by various tissues and cells. Since it can combine with chromosomes, IL-33 is regarded as an intracellular transcription repressor. Upon proinflammatory stimulation, it is released as an extracellular cytokine to function as an alarmin to dangerous signals. The IL-33 receptor is a heterodimer complex composed of ST2 and the IL-1 receptor accessory protein, the latter being conserved in other IL-1 family members. The IL-33/ST2 signaling pathway plays critical roles in inflammatory and immune diseases, as well as in central nervous system (CNS) diseases. Recently, there has been an increasing focus on IL-33, particularly on its production and functions in the CNS. The present review mainly focuses on progress in research on IL-33, especially its roles in the CNS.