The recent development of optical microscopy has brought about a more refined and sophisticated apparatus, such as a confocal laser scanning microscopy (CLSM), which was originally described by Minsky (1957), and ever since has been applied to the field of medical biology. In early experiments, only fluorescent signals were detectable by CLSM. However, recent innovations have prompted the possible visualization of non-fluorescent signals such as horseradish peroxidase (HRP) and diaminobenzidine (DAB) signals by CLSM. Moreover, the combination of CLSM and image analysis system (IAS) has enabled us successfully to visualize subcellular organelles three dimensionally (3D) in routinely processed optical microscopic specimens.In the present review, we applied CLSM to the specimens prepared for optical microscopy, and demonstrated the intracellular identification of subcellular organelles and protein, which were comparable to those observed under electron microscopy. We also applied this microscopy to the observation of tumor angiogenesis and microvessel environment of tumor cells. Both the visualization of subcellular organelles, mRNA and protein products, and 3D images of microvessel environment of tumor cells are discussed in this review.