Since the commercialization of the first genetically modified (GM) crop in 1996, the amount of arable land dedicated to the production of GM feed has increased significantly. Despite widespread adoption of GM foods and feeds, public perception of their safety remains mixed. To provide consumers the opportunity for choice, some countries have adopted mandatory labeling of GM foods and feeds when their adventitious presence exceeds a defined threshold percentage. Methods for detecting and quantifying GM plants in feeds include protein- and DNA-based assays, but their sensitivity may be influenced by the techniques used in feed processing. Interest in the consumption of transgenic protein and DNA has prompted investigations of their fate within the gastrointestinal tract of livestock and the potential to which transgenes or their products may be incorporated into tissues. Transgenic protein has not been detected in any animal tissues or products. Fragments of DNA from endogenous, high-copy number chloroplast genes from plants have been detected in poultry, pig and ruminant tissues. Low-copy endogenous and transgenic DNA in animal tissues have been detected but to a lesser extent than high-copy genes. Current research suggests that the passage of dietary DNA fragments across the intestinal wall is a natural physiological event, the likelihood of which is dependent on their concentration in the feed. To date, the transgenic traits approved for expression in crops used as feeds have not posed a safety concern for livestock.