Dynamic post-translational modifications (PTMs) regulate and diversify protein properties and cellular behaviors. Real-time monitoring of these modifications has been made possible with biosensors based on fluorescent proteins (FPs) and fluorescence resonance energy transfer (FRET), which can provide spatiotemporal information of PTMs with little perturbation to the cellular environment. In this review, we highlight available fluorescent biosensors applicable to detect PTMs in living cells and how they have shed light on biological questions that have been difficult to address otherwise. In addition, we also provide discussions about various engineering strategies for overcoming potential challenges associated with the development and application of such biosensors.