It is recognized that any risk assessment of genetically modified plants (GMP) is only as good as our state of scientific knowledge at the time it was conducted.  Pre-market biosafety studies can only address issues such as the potential adverse effects on non-target organisms for a limited number of species under limited environmental conditions.  Thus, an additional concept called 'monitoring' has been added to the EU legislation to address any remaining uncertainty. Monitoring can be defined as the systematic measurement of variables and processes over time.  It assumes that there are specific reasons to collect such data, e.g., to ensure that certain standards or conditions are being met or to examine potential changes with respect to certain baselines.  Against this background, it is essential to identify the type of effects or variables to be monitored, an appropriate time period for measurements and, importantly, the tools and systems to measure them.  Monitoring results, however, may lead to adjustments of certain parts of the original monitoring plan or they may be important in the development of further research (e.g., for plant gene flow consequences [Bartsch et al., 2003]).