In this talk I will describe our ongoing work in the development of Microwave Kinetic Inductance Detectors (MKIDs) for optical and infra-red astronomy. These super-conducting devices represent an important step towards the development of the 'ultimate detector'; one that can measure the position, energy and arrival time of a photon. I will describe the operating principles of the devices and their current status. I will describe ARCONS, the first MKID-based optical/IR instrument and the results of our recent commissioning run at the Palomar observatory. Finally, I will discuss the future promise of the technology, focussing on its use in the field of time domain astrophysics.