The purpose of this book is to provide basic reference material to facilitate the further development and application of the intracellular staining techniques which originated with the introduction of Procion dye as a neurocellular stain. We had sev­ eral specific objectives in compiling these chapters. First, we recognized that both the techniques and results of intracellular staining have been derived from a variety of preparations and published in diverse journals. Consequently, we tried to make this information more readily available by incorporating several reviews by original authors. This should provide the reader with a knowledge of the kinds of neurobiological prob­ lems for which intracellular staining has already been employed. A second objective was to facilitate extension of these methods to a wider variety of problems. To this end there are chapters dealing with Procion dye as a marker for ultrastructural investi­ gations, the use of cobalt as a staining compound and the implementation of computer techniques for quantitative studies of neuronal relationships. As background for further extension of these methods this volume contains material on the history of the discovery of Procion dyes as intracellular stains, the chemistry of these dyes, and fundamentals of fluorescence microscopy. Our final specific objective was to present details of technical procedures that were not readily available in the literature.