Combining general relativity and quantum theory: points of conflict and contact

Issues related to bringing together the principles of general relativity and quantum theory are discussed. After briefly summarizing the points of conflict between the two formalisms I focus on four specific themes in which some contact has been established in the past between general relativity and quantum field theory: (i) the role of the Planck length in the microstructure of spacetime, (ii) the role of quantum effects in cosmology and the origin of the universe, (iii) the thermodynamics of spacetimes with horizons, and especially the concept of entropy related to spacetime geometry, (iv) the problem of the cosmological constant.