Glia represent the majority of cells in the nervous system. Originally believed to be passive cells that support the neurons that transmit electrical signals, glial cells are now known to play a far more active part in neural development and function than previously appreciated. Written and edited by experts in the field, this new collection from Cold Spring Harbor Perspectives in Biology examines recent progress in our understanding of the major classes of glial cells―astrocytes, oligodendrocytes, Schwann cells, and microglia. The contributors review how technical advances and the use of worm, fly, and fish model systems are providing fascinating insights into the roles of glia in normal physiology and disease. They discuss how glia control synapse formation and plasticity, regulate blood flow, and organize axonal domains. They describe how glia are formed and generated from precursor cells during development. Other chapters tackle the roles of glial cells in myelination, connections with the lymphatic system, and neuroimmune interactions. The authors also discuss the functions of glia in tissue repair and regeneration, as well as disease processes such as glioma, demyelinating diseases, and neurodegenerative conditions. This volume is therefore a valuable reference for neuroscientists and biologists wishing to understand how these pathologies arise.