The growing popularity of the systems concept among agricultural scientists stems from the pragmatic desire to effectively understand the complex, non-linear processes the determine the success of agriculture in meeting multiple goals. Use of systems approaches in agricultural research began in the 1970's within several different agricultural sub-disciplines to achieve a variety of goals. As a result, the immense literature that encompasses agricultural systems research has not developed as an integrated body of knowledge but as parallel, independent literatures. The emergence of the various schools of systems analysis within agriculture has been driven by a number of factors, including 1) the realization that mainstream agricultural research and extension failed to serve small-scale, resource limited farmers in developing countries, 2) the evidence that industrial agriculture was damaging the environment 3) the emergence of ecosystem ecology as a field of study, and 4) the desire to develop more biologically-based production systems that could maintain yields while reducing environmental impacts. In spite of the disparate origins of systems approaches in agriculture, they have in common the central core of systems research. They are all based on the recognition that 1) agricultural systems are complex, 2) interaction among components determines characteristics of the system and 3) because of this complexity, intact agricultural systems need to be studied as a whole. However, because systems approaches were developed separately in response to different problems within agriculture, they each have distinct conceptual frameworks and emphasize different aspects of agricultural systems. Acknowledging the multiple roots of systems research allows us to understand the discrepancies that are evident in the agricultural systems literature and clarifies the need for some consistency in the way agricultural systems are conceptualized. These distinct approaches to agricultural systems research have complementary strengths which can be assembled into a comprehensive approach to agricultural systems research.