‘Biological control’ refers to the practice of controlling invasive pest populations by introducing their natural enemies into an ecosystem. Although biological control can reduce reliance on toxic chemicals and protect natural ecosystems, this approach is not without its challenges. Dr Peter McEvoy and his colleagues at Oregon State University discovered that certain biological control organisms show unexpectedly fast rates of evolution, which can lead to unforeseen impacts on ecosystems and agriculture. These scientists believe that it is time to develop an all-embracing theory to help assess the evolutionary potential of biological control organisms that may influence the efficacy and safety of future introduction programs.