Demeke, T. and Perry, D. J. 2014. Low level presence of unapproved biotech materials: Current status and capability of DNA-based detection methods. Can. J. Plant Sci. 94: 497-507. In agricultural biotechnology, low level presence (LLP) of recombinant DNA plant material is defined as the unintended presence of trace levels of a specific genetically engineered (GE) or biotechnology-derived material which in most instances has been authorized for use as food or feed in at least one country. Asynchronous authorizations of GE products have prompted testing for the GE content in an assortment of agricultural products for the purpose of facilitating international grain trade. Low level presence of some unauthorized GE materials identified in non-GE grains, oilseeds and food stuffs has negatively impacted grain trade. Other factors contributing to a negative impact on grain trade due to LLP of GE material include zero tolerance policies and slow regulatory approval processes for some countries. This element alone heightens the need for accurate, reliable and cost-effective detection methods. As the number of biotech events increases, the challenge of handling LLP of unapproved GE materials poses an even greater challenge. Polymerase chain reaction (PCR) is widely used for detection and quantification of GE events. Accuracy of PCR-based testing of GE events is affected by variation in sampling, sample preparation and various confounders associated with testing methods. Challenges when using PCR detection and quantification methods for the detection of LLP of GE events are the focus of this review as well as background information and recent examples of occurrence and suggestions to mitigate LLP as it relates to GE materials in grain trade.