It is now established that neuroglia are the intimate partners of neurons and that neuronal function is a result of neuron-glia interrelations at several levels of organization. The literature shows that the study of phylogeny has contributed a deeper understand- ing of the complex functions of the neuroglia and the neuron-glia unit. It is the purpose of Neuron-Glia Interrelations During Phylog- eny: I. Phylogeny and Ontogeny of Glial Cells, as well as its compan- ion volume Neuron-Glia Interrelations During Phylogeny: II. Plasticity and Regeneration, to present to the scientific community a broad spectrum of information on neuroglia through phylog- eny and ontogeny, the focus of this volume. In view of the role of neuroglia in plasticity and regeneration, the companion volume will cover this aspect of neuroglia during phylogeny. Neuron-Glia Interrelations During Phylogeny: I. Phylogeny and Ontogeny of Glial Cells begins with the elegant chapter "Glial Types, Gliogenesis, and Extracellular Matrix in Mammalian CNS" by Amico Bignami, to whom this volume is dedicated. He was one of the pioneers in describing gliogenesis and this chapter brings together everything we know today on this critical topic. It also includes the latest views of Bignami on the role of extracellular matrix in gliogenesis and glial functions. "Evolution of Astrocytes in the Vertebrate CNS" by Suarez et al. complements and extends the information in Bignami's chapter by including ependymal astrocytes.