Changes in conformation are an important regulatory mechanism for a wide variety of proteins. Proteins whose activity must change in response to external stimuli often undergo dramatic changes in their tertiary structure in a temporally and spatially coordinated manner, resulting in a change in enzymatic activity or in the profile of binding partners. To understand how these proteins function, it is critically important to be able to monitor the timing and subcellular localization of these conformational changes, preferably in a quantitative manner and in the context of a living cell. Unfortunately, there is a dearth of experimental techniques that can detect changes in conformation directly. In this chapter, we describe an approach that takes advantage of fluorescence resonance energy transfer (FRET), a well-known physical phenomenon between a spectrally compatible pair of fluorescent molecules, which is exquisitely sensitive to the distance between them. Combined with the use of proteins of the green fluorescent protein (GFP) family, this approach can be used to detect changes in protein conformation in vitro and in vivo effectively.