Phage therapy—application of bacteria-specific viruses to reduce densities of pathogenic or nuisance bacteria— is a two-step process involving phage penetration to target bacteria followed by bacteria killing. Any analysis of these steps is inherently ecological as they represent phage-environment interactions, i.e., between phages and bacteria as well as between phages and body tissues. In considering phages more generically, as selectively toxic antibacterial agents employed to treat bacterial infections, the term “ecology” may be fairly cleanly replaced with the term “pharmacology”. Pharmacology, in turn, may be distinguished into two major components: pharmacokinetics and pharmacodynamics. Pharmacokinetics is explicitly a description of the bodys impact on a drug (e.g., movement through and between body compartments) whereas pharmacodynamics is a description of a drugs impact on the body. “Body” includes both body tissues and microbial flora, so an important component of antibacterial pharmacodynamics is inhibition of the growth of target bacteria. Our guiding premise is that phage therapy may be rationally improved through a better understanding of phage pharmacokinetics and pharmacodynamics. Our primary conclusions are (i) that the principle advantages of phages, over antibiotics, are the formers relative safety and ease of discovery; (ii) that phage therapy efficacy is highly dependent on attaining relatively high phage “killing titers”; (iii) that attainment of sufficient titers solely via in situ phage replication should, in some or many circumstances, not be counted upon; and (iv) that phage replication nonetheless may provide a “margin of safety” toward attaining phage therapy efficacy.