Search for the end of the cosmic ray energy spectrum

The title I was asked to speak about expresses an idea that occurred rather recently in the history of cosmic ray studies. I argue that the idea of a possible end of the cosmic ray energy spectrum came into being after a sequence of three rapid advances in knowledge which I describe, calling them ‘breakthroughs.’ I suggest that the present workshop be regarded as a step toward a fourth breakthrough. I argue that this may occur through application of the Space Airwatch concept—the earth atmosphere as target and signal generator—as embodied in the NASA OWL project.