A plant breeder discovers his experimental crops have been “contaminated” with genes from a neighboring field. New nasty weeds sometimes evolve directly from natural crosses between domesticated species and their wild relatives. A rare plant is threatened due to its small population size and restricted range. What do all these situations have in common? They illustrate the important role of gene flow among populations and its potential consequences. Although gene flow was recognized by a few scientists as a significant evolutionary force as early as the 1940s, its relative role in maintaining a species’ genetic integrity and/or its diversity has been debated over the decades, vacillating from trivial to critical.