Crop rotations are defined in terms of a number of crops organized in a specific order over a defined period of time, with specific spatial arrangements and distribution in a field and landscape, that ensures the best growing conditions for all crops. Crop rotation is thus a way to utilize the performance of a single crop in a way that contributes to the whole farming system. This chapter examines the principles of crop rotation, precrop effects in crop rotations, and the nutrient effects of legumes and other rotation crops. The chapter also considers the role of rotation crops in suppressing weeds, diseases and pests and studies rotations and crop yields, as well as the challenge of designing a crop rotation. The chapter looks at measuring and modelling crop rotations, nutrient and humus balances, crop rotations on stockless farms, intercropping and crop rotations in tropical and subtropical conditions.