1 October 2017 marks the start of ESO Period 100. To celebrate this centenary we look back at the evolution of observing time proposals at ESO. This article examines the way that science is facilitated by ESO and how this has evolved as new technologies mature in order to take advantage of new ideas from astronomers and engineers from across the ESO Member States and beyond. We look at how the first ESO observing periods were defined and how different the calls for proposals and proposal reviews were at that time. We then detail how these processes changed as the VLT started, showing how Service Mode has fundamentally changed how astronomy is being done on the VLT. Finally we look to the future, describing forthcoming instruments and experiments on ESO telescopes and at other facilities hosted onsite. We conclude by describing some of the challenges faced by ESO and the user community and how procedures will need to evolve further to accommodate these.