Black holes are important in linking microphysics with macrophysics, and those of the Planck mass must play a fundamental role in any marriage of quantum theory and general relativity. In particular, the Black Hole Uncertainty Principle correspondence posits a smooth transition between the Compton and Schwarzschild scales as a function of mass. This suggests that there is a duality between elementary particles below the Planck mass and black holes above it, with elementary particles being interpreted as sub-Planckian black holes. Higher dimensions may also elucidate the connection between quantum and classical physics, with higher-dimensional black holes relating to the birth of the Universe and the embedding space of general relativity providing a quasi-classical interpretation of some features of quantum theory. This may also elucidate the problem of the passage of time.