The absence of gene flow, genetic isolation, is frequently emphasized in conservation genetics. However, the presence of gene flow can play an equally important role in determining the genetic fate of populations. Here, I first review what is known of patterns of gene flow by pollen. Gene flow by pollen is often substantial among plant populations. I next review the expectations for gene flow patterns in the small populations typical of endangered species. Then, I consider what role gene flow can play in plant conservation genetics. Depending on the specific situation, such gene flow could be either beneficial or detrimental. Geographically disjunct populations might not always be as reproductively isolated as previously thought, and thereby less vulnerable to detrimental drift-based processes. On the other hand, conspecific or heterospecific hybridization may lead to extinction by outbreeding depression or genetic assimilation. Also, the field release of transgenic plants may lead to the escape of engineered genes by crop-wild plant hybridization. Such 'genetic pollution' could have profound effects on the fitness of wild species with the potential for disrupting natural communities. Gene flow can be an important force in plant conservation genetics, and its potential role should be considered in any plant conservation management program.