Biosensors are used in many fields to measure the concentration of analytes, both in a cellular context and in human samples for medical care. Here, we outline the design of two types of modular biosensors: SNAP-tag-based indicators with a Fluorescent Intramolecular Tether (SNIFITs) and LUCiferase-based Indicators of Drugs (LUCIDs). These semisynthetic biosensors quantitatively measure analyte concentrations in vitro and on cell surfaces by an intramolecular competitive mechanism. We provide an overview of how to design and apply SNIFITs and LUCIDs.