In this paper, I present a history of the development of diagnostic techniques for the chemical oxygen iodine laser (COIL). Several established optically based techniques have been applied to COIL including: visible and near infrared chemiluminescence, resonance absorption, and laser induced fluorescence. I trace the history of these developments using the diagnostic methods as the overall theme. In many cases a variant of an established diagnostic was used to probe for some key kinetic rate or mechanism. Indeed, the goal of developing the now well established COIL kinetic rate package was responsible for the introduction of new diagnostic methods. I discuss diagnostics both before and after the demonstration of the first COIL device.