 The predominant current-day meaning of genotype is some relevant part of the DNA passed to the organism by its parents. The phenotype is the physical and behavioral traits of the organism, for example, size and shape, metabolic activities, and patterns of movement. The distinction between them is especially important in evolutionary theory, where the survival and mating of organisms depends on their traits, but it is the DNA, held to be unaffected by the development of the traits over the life course, that is transmitted to the next generation. Philosophical discussion mostly now takes the predominant meanings as given, focusing instead on questions about the genotype-phenotype relationship. For example: How can DNA be construed as information for the processes of development of an organism’s traits? What is the causal status of DNA versus other influences in the organism’s development? (see entries on  inheritance systems  and  biological information  and also Griffiths &amp; Stotz 2013). Without dismissing the importance of such questions, the focus of this entry remains on the genotype-phenotype distinction. Given that discussion by philosophers of this issue has been minimal, this entry cannot take the standard form of a review of published debates. In order to help frame and orient readers’ conceptual inquiries, another approach is needed. The entry builds from the observations that the original meanings of genotype and phenotype and the distinction between them as given by Wilhelm Johannsen (1911) were quite different from the now predominant meaning (given above) and that different kinds of meanings coexist in Johannsen and up to the present. To make sense of those observations, Johannsen’s paper is recounted with special reference to practices or assumptions regarding control of biological materials and conditions. Subsequent developments are then described in relation to problems introduced by conceptualizing heredity on the basis of breeding experiments. This framing brings into play many areas of philosophical discussions, including the “New Experimentalism” in philosophy of biology (see entry on  experiment in biology),  abstraction (see entry on  abstract objects),  confirmation, ambiguity, Scientific Realism, descriptive versus normative approaches, and The Social Dimensions of Scientific Knowledge. At the same time, the framing helps open up questions that have not been well addressed in those discussions and has implications for issues that might seem settled to many who, when they invoke the genotype-phenotype distinction, are simply thinking DNA versus trait. 