Genetic modification encompasses a wide range of techniques that does "exactly what it says on the tin" - i.e. changes the genetic make-up of the cells of a given organisms which in turn affects the characteristics of that organism. In food, we generally mean things like GM Tomatoes where a gene that promotes ripening has been "switched off", producing tomatoes that stay fresh for longer. GM Maize (Corn) is another big issue - here a gene from a certainn bacteria that produces an insect toxin is transferred to this crop so that it "naturally" produces an insecticide, therefore (in theory at least) giving a crop that is more resistant to insects, producing a better yield and requiring less chemicals to be sprayed on it. As far as scientists can see, such manipulations have no effect on humans. However, it is debated how well we can ever know this. Other points of controversy are whether we have the right to play God with nature, the potential environmental knock-ons (once the gene is spread in the wild it is uncontrollable) and the role of the big agricultural and biotech firms in controlling scientific research and their motives for it.