Why does a metal–superconductor junction have a resistance?

This is a tutorial article based on a lecture delivered in June 1999 at the NATO Advanced Study Institute in Ankara. The phenomenon of Andreev reflection is introduced as the electronic analogue of optical phase-conjugation. In the optical problem, a disordered medium backed by a phase-conjugating mirror can become completely transparent. Yet, a disordered metal connected to a superconductor has the same resistance as in the normal state. The resolution of this paradox teaches us a fundamental difference between phase conjugation of light and electrons. To be published in Quantum Mesoscopic Phenomena and Mesoscopic Devices in Microelectronics, edited by I. O. Kulik and R. Ellialtioglu (Kluwer, Dordrecht).