Fluorescent protein biosensors have become an important tool for neuroscience research. Highly optimized biosensors for Ca2+ are widely used to study neuronal activity in live animals. Underlying the impressive functionality of these fluorescent probes has been a tremendous research and engineering effort to understand their function and engineer new variants with improved performance. While the potential for fluorescent biosensors to enable new directions in biological research is clear, biosensors for other targets have not reached the same widespread usage or impact. Underlying this is a combination of factors including the difficulty of generating new and improved biosensors and the challenge of identifying informative analytes to measure.