The modeling of insect phenology has been an important component of IPM decision support systems for several decades. This chapter describes general concepts and methods of the systems approach using applied phenology modeling as an example. The authors outline the steps of model construction from a systems perspective, and then discuss funding challenges, types of resources, and major sources of error in phenology model development and implementation. The chapter also compares some of the trade-offs and pitfalls of using linear and non-linear approaches to modeling temperature-development data. It provides an example of using the systems approach by presenting a life cycle systems model for the spotted wing drosophila, Drosophila suzukii, a major fruit crop pest in the US and Europe. The model can serve as a way to summarize current knowledge and potential best management practices that can guide the development of IPM programs.