 Discussions with many colleagues, including those who saw the genotype-phenotype distinction to be a boring topic, stimulated the revision of this entry. Comments on drafts by Jonathan Kaplan, Barbara Mawn, Rasmus Winther, and an anonymous reviewer also helped. Some passages in this entry have been carried over from the previous SEP version by Lewontin; some sentences have been adapted from Taylor (1987, 2003, 2014, 2015). 