Background) Phage therapy is the use of bacterial viruses (phages) to treat bacterial 1 infections. Phages lack the broad host ranges of antibiotics, so individual phages are often used with 2 no prior history of use in treatment. Therapeutic phages are thus often chosen based on limited 3 criteria, sometimes merely an ability to plate on the pathogenic bacterium. It is possible that better 4 treatment outcomes might be obtained from an informed choice of phages. Here we consider whether 5 phages used to treat the bacterial infection in a patient might specifically evolve to improve treatment. 6 Phages recovered from the patient could then serve as a source of improved phages or cocktails for 7 use on subsequent patients. (Methods) With the aid of mathematical and computational models, we 8 explore this possibility for four phage properties expected to promote therapeutic success: in vivo 9 growth, phage decay rate, overcoming resistant bacteria, and enzyme activity to degrade protective 10 bacterial layers. (Results) Phage evolution only sometimes works in favor of treatment, and even in 11 those cases, intrinsic phage dynamics in the patient are usually not ideal. An informed use of phages 12is invariably superior to reliance on within-host evolution and dynamics, although the extent of this 13 benefit varies with the application. 14