Since the inception of PE&RC in 1994 the unifying systems approach has been a major binding element of the PE&RC research agenda. In this approach, the system performance is explained from insights into the basic physical, chemical, physiological and ecological processes taking place in various systems in time and space. Modelling and experimentation/observation are two approaches that are inextricably connected and of mutual importance to investigate and exploit the occurring and emerging phenomena in these systems. In the 1960s, Prof. Cees de Wit was one of the first using this scientific approach. Nowadays, modelling activities within the research programme of PE&RC are common practice. PE&RC provides opportunities to increase coherence between modelling activities and thus strengthen the role of the Graduate School as an international centre of excellence in (agro-)ecological modelling.
The systems approach is applied at different temporal and spatial scales and is especially useful for studying their interrelationships. Spatial scales vary from the organism (such as the plant), population, (agro-)ecosystem and region to supra region taking into account management units such as farms. Time scales vary from seconds, days, months, to years. Strong emphasis is given to basic as well as to problem-oriented research, which by definition is interdisciplinary. The main research efforts focus on the plant, the crop, the field, vegetation or ecological zones, often taking one particular scale-time frame as the bounding conditions for research. The interaction with social sciences, especially at the higher integration levels is of utmost importance.