This book summarizes recent scientific results on organic nanoparticles in view of the observation, measurement, and manipulation of single particles. This approach makes it possible to extract the nature of organic nanopar­ ticles without considering the averaged information ofthe nanoparticles over distributions of size, shape, inner structure, and environment. It is based on recent progress in laser, microscope, and optical detection systems. Various kinds of new methodology, theory, analysis, and preparation of organic na­ noparticles have been developed and applied. Novel phenomena, properties, characteristics, and functionality have been explored and revealed. Such stu­ dies on the chemistry and physics ofnanoparticles is bridging our gaps in the understanding of single molecules, atoms, and bulk materials. Molecular pictures are particularly useful for predicting, explaining, and designing the the physical and chemical properties of organic nanoparticles. This is especially the case in light of the availability of more than 15 million kinds of molecules for synthesis. This approach is opening new aspects of nanoscience and nanotechnology which can never be attained by studies on nanoparticles of metals and semiconductors.