With nearly all of our billions of wireless links having no line-of-sight between the transmitter and receiver, multipath propagation mechanisms, and their accurate modelling, are coming under revitalized scrutiny. This paper reviews diffraction and how it models transmission around blind corners even when there are no supporting surface currents on the corner structure. From the context of defining diffraction as the difference between geometric optics and reality, the basic equations are reviewed and compared to results from numerical experiments (simulations) and physical experiments, to test the theory. The simulations are from a finite-difference time-domain type of approach, which can be expected to work reasonably well, whereas the method of moments, because it uses surface current sources (electric or magnetic), cannot be expected to demonstrate diffraction for many situations. The applications range from radiosity and simulation tools for architectural lighting and animation, to accurate coherent propagation prediction around multiple, canonical baffles.