Adaptive cancer therapy aims to delay cancer progression by exploiting competition between drug-sensitive and -resistant cells in the tumor. Drug dosing is adapted in a patient-specific fashion to maintain drug-sensitive cells that competitively suppress resistance (blue). This is in contrast to standard-of-care cancer treatment regimens that maximize cell kill and thereby cause the rapid competitive release of drug-resistant cells (orange). But, when will adaptive therapy work? Shown is a collage of so-called “phase plane” visualizations of a mathematical model with which the authors address this question. Each triangle represents a different parameterization. It was found that resource availability, resistance fraction, resistance cost, and cellular turnover integrate to modulate intratumoral competition. For details, see article by Strobl and colleagues on page 1135.