Intravital imaging techniques are powerful tools in the study of tumor pathophysiology. These techniques can be used to measure vascular parameters (e.g., angiogenesis, hemodynamics, and leukocyte-endothelial interactions) in tumors and normal vascular networks in mice. This protocol describes the use of single-photon microscopy and multiphoton laser-scanning microscopy to follow the migration of Rhodamine-6G, a fluorescently labeled tracer, following injection into the tail vein of a mouse. The Rhodamine-6G enters the circulatory system and labels leukocytes. It is possible to monitor changes in the interactions between leukocytes and the endothelium by determining the numbers of rolling and adhering leukocytes as well as the total flux of these cells.