If we had the technology than can measure specific biomolecules inside of bodies in "real time", it would transform medicine by providing clinicians with a valuable window into patients' health and their response to therapies. Unfortunately, such real-time measurements are currently not possible, except for a handful of molecules such as glucose. In this presentation, we will discuss recent advancements in "real-time biosensors." As examples, we will present our group's work on continuously measuring anti-cancer and anti-bacterial drugs in live animals with unprecedented time-resolution. Finally, we will discuss the exciting possibility of actively controlling these molecules inside of patients using closed-loop feedback control.