A thin superconducting film in the transition between its superconducting and normal states makes an ultra-sensitive thermometer, capable of measuring the energy of individual x-ray and gamma ray photons. The transition edge sensor, like the superconducting transmission line and electromagnet, is not a new idea. The notion of using the superconducting transition to measure the tiny amount of heat imparted by a single photon dates back to the late 1930s. But, like so many of the early ideas for applications of superconductivity, it would take decades of innovation in theory, electronics, microfabrication, and cryogenics for the transition edge sensor (TES) to realize its potential. Now, the TES is used to measure radiation across the electromagnetic spectrum, from microwaves to gamma rays. By arraying hundreds or even thousands of sensors, we can build spectrometers with an unparalleled combination of precision and efficiency. TES arrays are now ubiquitous in ground-based observatories that study the cosmic microwave background. A TES spectrometer being designed for the x-ray satellite Athena will probe the composition of faint, faraway galaxy clusters. Today, a TES array at the Stanford Synchrotron Radiation Light Source measures light emitted by delicate biological molecules as they undergo complex chemical reactions. In fact, with a TES array many experiments that were once only possible at a synchrotron, one of the most powerful x-ray sources in the world, can now be conducted on a bench-top in the lab. We have entered an era where we dont just study superconductivity, we use the superconducting phase transition as a tool for scientific discovery.