In the future there will be a very strong competition between arable land use for phytogenic biomass production for feed/food, fuel, fibre and other industrial materials, as well as for settlements and natural conservation areas because of the growing population and limited natural resources. Therefore plants with high and stable yields, and requiring low external inputs (low input varieties) should be the main aim of plant breeding. In addition to traditional breeding, plant biotechnology seems to have the potential to contribute to this objective. Nutritional and safety studies with feed/food made from such modified plants are one of the most important prerequisite for public acceptance, and to improve knowledge in the feed/food sciences. The first step for the nutritional and safety assessment of such modified plants is the compositional analysis of potential feed/food, including the newly expressed proteins and other new constituents, and its comparison with conventional counterparts. In vitro studies and experiments with laboratory animals comprise the next steps of the assessment. About 70–90 % of the harvested biomass from genetically modified plants (GMPs) is consumed by food producing animals. Therefore, feeding studies with target animals are of special concern for nutritional assessment, and these are considered in more detail in the present paper. Up to now most studies have been done with GMPs of the 1st generation (plants with input traits, but without substantial changes in composition). Other experimental designs for nutritional and safety assessments are recommended for GMPs with output traits or with substantial changes in composition (plants of the 2nd generation).