Mathematical models of within-host processes can be nested within and higher-scale, epidemiological or between-host processes. Nested approaches are useful, in part, because they force biologists to think about and describe how biological processes interact between scales. One important insight from nested models is that, under simple scenarios, an understanding of how within- and between-host selection on pathogen replication rates interact. I will present an outline of these types of models and will suggest that a similar approach could be developed to aid our understanding the short and long-term evolutionary implications of the trade-offs involved in the evolution of drug resistance.