  This is an introduction to quantum gravity, aimed at a fairly general
audience and concentrating on what have historically two main approaches to
quantum gravity: the covariant and canonical programs (string theory is not
covered). The quantization of gravity is discussed by analogy with the
quantization of the electromagnetic field. The conceptual and technical
problems of both approaches are discussed, and the paper concludes with a
discussion of evidence for quantum gravity from the rest of physics.
  The paper assumes some familiarity with non-relativistic quantum mechanics,
special relativity, and the Lagrangian and Hamiltonian formulations of
classical mechanics; some experience with classical field theory, quantum
electrodynamics and the gauge principle in electromagnetism might be helpful
but is not required. No knowledge of general relativity or of quantum field
theory in general is assumed.
