 I wish to think many people discussions with whom have shaped my understanding of decoherence over the years, in particular Marcus Appleby, Elise Crull, Matthew Donald, Beatrice Filkin, Meir Hemmo, Wayne Myrvold, Josh Rosaler, Simon Saunders, Max Schlosshauer, David Wallace, and Wojtek Zurek. For more recent discussions, correspondence and suggestions relating to this article I wish to thank Valia Allori, Maaneli Derakhshani, Bob Griffiths, Ronnie Hermens, Peter Holland, Martin Jones, Brian Josephson, Tony Leggett, Hans Primas, Alberto Rimini, Philip Stamp, and Bill Unruh. I also gratefully acknowledge my debt to Steve Savitt and Philip Stamp for an invitation to talk at the University of British Columbia, to Claudius Gros for an invitation to talk at the University of the Saarland, and for the opportunities for discussion arising from those talks. Finally I wish to thank the following: the referee for this entry, David Wallace, for his clear and constructive commentary; my former subject editor, John Norton, who corresponded with me extensively over a previous version of part of the material and whose suggestions I took to heart; my current subject editor, Wayne Myrvold; our editor-in-chief, Ed Zalta, and all his staff, for their saintly patience; and my late and lamented friend, Rob Clifton, who invited me to write on this topic in the first place.