Evolution can be thought of as a process in which genotypic changes allow phenotype to track the environment. The exact nature of this process depends on the details of the interactions between genotype and phenotype, and between phenotype and the environment – both of which are key problems in biology. In the case of HIV, the HLA type of the host is an important environmental variable that can affect the evolution of viral genotype, and understanding this effect can lead to important insight. However, constructing viable models that incorporate phenotype and environment into the description of genotype evolution is exceedingly challenging. In this talk I will discuss two different approaches that begin to address this problem.