 I am grateful to Stephen Barnett and especially Jeremy Butterfield for close advice concerning the physics and philosophy of physics dealt with in this entry. For further discussion of the material in this entry, I am grateful to Helen Beebee, Chris Daly, Bill Demopoulos, Scott Dixon, Cian Dorr, Kit Fine, Berta Grimau, Herbert Hochberg, Frederique Janssen-Lauret, Sam Lebens, Joop Leo, Stephan Leuenberger, Kevin Mulligan, Francesco Orilia, Gary Ostertag, Laurie Paul, Jonathan Schaffer, Stewart Shapiro, Peter Simons, Tim Williamson, and Ed Zalta. The material in this entry formed the basis for a course on relations at Glasgow University and I’d like to thank the students for their many helpful comments and suggestions.