Crop modeling is an integrated discipline of agricultural sciences that contributes to agricultural research, including the areas of agronomic management, precision agriculture, environmental impacts and externalities, input efficiency, managing climate variability and seasonal forecasting, breeding, climate change impacts and adaptations, and agricultural policy. More recently, the crop modeling community has begun to exchange models, model routines, and data in coordinated and protocol-based crop model inter-comparison and improvement projects. In this chapter, the authors summarize crop model inter-comparison studies and their contribution to sustainable agriculture and discuss gaps and trends in crop simulation model inter-comparisons and improvements. Following an introduction, Sections 2–5 cover crop model inter-comparison, crop model uncertainty, crop model improvement, and crop model application, respectively. A case study on the Agricultural Model Inter-comparison and Improvement Project for Wheat (AgMIP-Wheat) is also included. The chapter concludes with a review of future trends in research.