To track the activity of cellular signaling molecules within the endogenous cellular environment, researchers have developed a diverse set of genetically encodable fluorescent biosensors. These sensors, which can be targeted to specific subcellular regions to monitor specific pools of a given signaling molecule in real time, rely upon conformational changes in a sensor domain to alter the photophysical properties of green fluorescent protein (GFP) family members. In this introductory chapter, we first discuss the properties of GFP family members before turning our attention to the design and application of genetically encodable fluorescent biosensors to live cell imaging.