Quantum Histories and Quantum Complementarity

We discuss the origin of quantum complementarity using Feynman's sum-over-histories approach to Quantum Mechanics (QM). We analyze two experimental setups introduced by W. G. Unruh and S. Afshar and show how one should consistently calculate the existence of which-way information using interfering quantum histories. Then we discuss the possible problems associated with the notion of which way information and provide an explanation why Englert-Greenberger duality relation cannot be violated experimentally if the standard QM postulates are accepted.