Functional foods are routinely cited as a top trend shaping the future of the food industry, but what are they, really? A grab-bag term that can include everything from genetically modified tomatoes to cholesterol-lowering margarines to juices laced with ginseng and echinacea, functional foods can be devilishly difficult to define. After all, what food isn't in some way "functional?" Maybe that's why estimates of functional food sales in the United States range from US$4bn to more than US$200bn.