The commercial use of genetically modified (GM) organisms is regulated in the EU by law. Thus, monitoring the environmental effects of GM organisms after placement on the market is a mandatory task of the respective consent holder. Since many relevant monitoring procedures lack standardisation, the Association of German Engineers (VDI) has commissioned expert groups with the development of guidelines covering appropriate methodologies. As part of this project, the VDI Guideline 4330 Part 10 was set up (Bleeker et al. 2011) describing a standardised procedure for floristic mapping of spontaneously occurring (non-cultivated) GM crops, their wild potential crossing partners and their hybrid offspring. Areas to be mapped are those where such plants are expected to be found, e.g. on former fields and in the vicinity of current or former fields of GM plants. In the case of transportation, processing or use of GM plants as animal feed, these are areas surrounding the processing, storage, handling and usage facilities, including access routes to and from the facilities. The concept of adverse environmental effects caused by the dispersal and outcrossing of GM plants is briefly introduced. The necessity of floristic mapping in the context of post-market environmental monitoring of GM plants is demonstrated taking oilseed rape as an example. The development of the Guideline VDI 4330 Part 10 is described and its contents are summarised. An important conclusion on the relevance and efficiency of the floristic mapping method is that strict standardisation ensures a high level of EU wide reproducibility and comparability of the results.