Years after the commercialisation of genetically modified (GM) plants, the debate about their potential risks and benefits is still ongoing. Surprisingly little is known about interactions between GM plants and their environment. In this book, I present glasshouse and field experiments that shed light on the ecological behaviour of GM wheat carrying fungal resistance transgenes. We found that these GM lines had poor performance and that the transgenes caused unexpected effects in the field despite high performance and no unexpected effects in the glasshouse. Interestingly, even GM lines with identical transgenes differed among each other. This suggests that transgene expression levels influence the ecological behaviour of GM crops more than we would expect from the simple presence of additional genes. It is therefore essential to assess risks and benefits of GM lines on a case-by-case basis and when possible in the field. Finally, we could demonstrate positive diversity effects - GM lines performed better when grown in mixtures than in monoculture. Such basic ecological knowledge can potentially improve sustainability and productivity of agricultural systems.