Peritoneal metastasis is the most important prognostic factor for gastric and ovarian cancer patients. The protocol in this chapter presents in vivo imaging procedures capable of examining the development of peritoneal metastasis from the micrometastasis stage to the advanced stage. We also describe in vivo imaging procedures for monitoring of antimetastatic agents in nude mice. In vivo imaging systems described consist of green fluorescent protein (GFP) or red fluorescent protein (DsRed) gene-tagged metastatic cancer cell lines and a handy detection device for GFP (or DsRed). This system allows both external, noninvasive, and real-time monitoring of the therapeutic effects of drugs within the animal facility and internal visualization of micrometastases at the cellular level using fluorescence microscopy. Selection of micrometastasis-positive mice and timing of drug administration after injection of tumor cells is critical for accurate evaluation of anti-metastatic efficacy. The present real-time fluorescence imaging system using GFP- and DsRed-tagged metastasis models makes it possible to overcome these problems and therefore is an indispensable tool for preclinical metastasis research and drug discovery.