Green fluorescent protein (GFP) is an ideal reporter in in vivo studies. Flow cytometry and fluorescent microscopy are two conventional tools to detect the GFP signal; flow cytometry is an effective and sensitive technique to quantitatively analyze fluorescent intensity, while fluorescent microscopy can visualize the subcellular location and expression of GFP. In this chapter, we describe a method using GFP as a reporter under the control of a target gene promoter. The system allows measurement of the levels of target gene expression by both fluorescent microscopy and flow cytometry. This method can be applied not only to dissect the target gene promoter but also as a sensor to detect environmental pollutants.