Measurements of nitrous oxide (N2O) emissions from agriculture are essential for understanding the complex soil-crop-climate processes, but there are practical and economic limits to the spatial and temporal extent over which measurements can be made. Therefore, N2O models have an important role to play. As models are comparatively cheap to run, they can be used to extrapolate field measurements to regional or national scale, simulate emissions over long time periods, or to run scenarios to compare mitigation practices. Process-based models can also be used as an aid to understanding the underlying processes as they can simulate feedbacks and interactions that can be difficult to distinguish in the field. In this paper, we discuss common modelling techniques such as calibration/validation, assessing model fit, sensitivity analysis and uncertainty assessment. We also review examples of N2O modelling for different purposes and describe some of the commonly used N2O models.