In vivo and ex vivo fluorescence video microscopy used to be a well-established method in life science with a variety of applications, such as in inflammation or cancer research. In this book chapter, we describe a model of in vivo fluorescence microscopy of the rat's lung with the exclusive advantage of qualitative and quantitative in vivo analysis of cell adhesion within the complex microenvironment of the ventilated and perfused lung. Observation can include real-time, time-lapse, or fast-motion analysis. In our laboratory, we have used the model for qualitative and quantitative real-time analyses of metastatic colon cancer cell adhesion within the rat's pulmonary microcirculation. Using some modifications in another series, we have also applied the model to analyze thrombocyte and leucocyte adhesion within the pulmonary capillaries in experimental sepsis. For interventional studies, injected cells or animals may be pretreated with various reagents or drugs for further analysis of adhesion molecules involved in tumor cell-endothelial cell interactions.