The meaning of entropy in gravitational physics has been mysterious since the beginnings of black hole thermodynamics in the work of Bekenstein and Hawking approximately half a century ago.   By coincidence, another development that occurred at almost the same time was the work of Takesaki and Connes using the ``crossed product'' of a von Neumann algebra by its modular automorphism group to simplify the study of von Neumann algebras of Type III. In this lecture, I will explain in what sense the crossed product sheds light on the meaning of black hole entropy.