RNA interference (RNAi) is now widely used in plant biotechnology, both as a useful tool for discovering or validating gene functions as well as a quick way of engineering specific reductions in expression of chosen genes. Although the amazing popularity of RNAi as a biotechnology tool is certainly justified, the underlying biology is still being worked out and the relative advantages and disadvantages of the approach are only now becoming clear. Recent breakthroughs in elucidating the multiple pathways of RNA-based post-transcriptional control and preliminary results from the first large-scale uses of RNAi in plants will make it easier to gauge the usefulness of the technique. To fully capitalize on the potential of RNAi, we need to become better at predicting and controlling its effects.