Christopher Isham (born 28 April 1944), usually cited as Chris J. Isham, is a theoretical physicist at Imperial College London. His main research interests are quantum gravity and foundational studies in quantum theory. He was the inventor of an approach to temporal quantum logic called the HPO formalism, and has worked on loop quantum gravity and quantum geometrodynamics. Together with John Baez, Isham is known as a proponent of the utility of category theory in theoretical physics.