GM foods first appeared in the 1990s. The dangers of genetically modifying foods are not yet fully understood or apparent. There are undoubtedly many scare stories, which you can find easily by scouring the internet, just as there are benefits identified in modifying a crop so that (say) it produces better yields, or is more resistant to disease.

GM foods may be linked with allergies, cancers, and many other problems. You will almost certainly have eaten GM products without realising it: the commonest GM foods include soy, corn, and wheat, and it is estimated that in by the late 1990s 75% of US processed foods contained a GM ingredient.

There is now a much wider awareness of the GM issue. Many suppliers have consciously decided not to use GM ingredients, given the level of public concern.

Food containing GM ingredients has to be labelled if it contains more than 0.9% GM ingredients. However, if GM feed is given to animals the resultant cheese, milk or eggs do not have to be labelled. The only way to be certain you are not consuming small amounts of GM food is to buy organically certified food.

The main argument for GM – that it enables more crops to be produced in the parts of the world where food is desperately needed, are dismissed by leading charities. The main issues preventing crop production are lack of land, war and natural disasters, none of which are addressed by GM technology.

The real commercial reason for introducing them is that the companies manipulating the genes than have a patent on the crop, enabling them to licence and exploit it. They also produce the chemicals to treat their own specifically modified crop, giving them a further area for profit.

More reading:

Wikipedia

Friends of the Earth

Greenpeace