In the last decade multiphoton excitation microscopy has emerged as an important technique with ever increasing numbers of significant applications in the fields of biology, chemistry, physics, and medicine. This volume contains key papers on the following topics: developments of nonlinear optical spectroscopy and nonlinear scanning microscopy (SHG, CARS); theory and techniques of multiphoton excitation microscopy; development of laser sources; single-molecule studies; and applications to biology, cell biology, embryology and developmental biology, neuroscience, dermatology, and optical biopsy. A comprehensive bibliography follows the reprinted papers.