The key analytical challenge to developing agricultural system models is to predict the behavior of agricultural systems in the multiple dimensions that are relevant to their economic, environmental and social sustainability, under recent historically observed conditions and when they are modified to improve their performance. This chapter examines ex ante technology impact assessment, defined as forward-looking efforts at technology design and evaluation. Supported by a case study, the chapter discusses methods to combine process-based and economic production models to overcome some of the limitations of each approach. Finally, the chapter looks ahead to future research trends in this area, and provides detailed guidance on further reading.