In 1851, Heinrich Muller discovered what he called "radial fibers" and what we now call Muller cells, as the principal glial cells of the vertebrate retina. Later on, other glial cell types were found in the retina, including astrocytes, microglia, and even oligodendrocytes. It turned out that retinal glial cells are essential constituents of the tissue. For instance, Muller cells appear to constitute the "core" of columnar units of clonally and functionally related groups of neurons. Their primary function is to support neuronal functioning by guiding the light towards the photoreceptor cells, removing excess neurotransmitter molecules from extracellular space, and performing efficient clearance of excess extracellular potassium ions. The latter two functions are also crucial for neuronal survival and are coupled to water clearance which is also essential. Muller cells are capable of "sensing" neuronal activity and modifying it by the release of signal substances (gliotransmitters). In cases of retinal injuries the Muller cells become reactive, and all above-mentioned functions are impaired. However, such de-differentiated Muller cells may proliferate, and may even serve as stem cells for the regeneration of a damaged retina. As well as the Muller cells, retinal astrocytes and microglial cells are important players in retinal development and function. This book gives a comprehensive survey of the present knowledge on retinal glia."