Although the physiological principles involved in the growth of greenhouse and field crops are not basically different, the development of models for greenhouse crops to some extent has followed its own way. This is mainly due to the specific characteristics of the crops and of the greenhouse production systems involved. Many important greenhouse crops are multi-harvest crops, where the balance between vegetative and generative growth is an aspect of major concern to growers. Moreover, most products have a high water content and they are sold fresh. In food crops, taste is a valuable crop property. In ornamentals, shape and colour are important characteristics that put certain demands on the output of models. More generally, quality issues (e.g., shelf or vase life) often have to be approached in a different way than with field crops. Last but not least, the huge number of species is problematic for crop modellers in horticulture. Modern greenhouse production systems provide the grower with a highly advanced, but expensive system for controlling the aerial and root environments of the crop. Through this control system growers are able to control the production process in great detail. The high added value obtained in greenhouses and the high quality requirements go together with a great deal of human interference in the production process: either directly, by pruning and training, or indirectly, by using various organisms for pest control and pollination. Crop management and the interaction of pests and diseases with the crop are both aspects that make special demands on crop models. It is a major challenge for greenhouse growers to make the best possible use of the available options to achieve high productivity at the moment when products are required in the market. In addition, this must be accomplished while reducing the environmental impact by emissions of CO2, nutrients, and biocides and at minimum cost. To optimise greenhouse production systems, crop models are needed. But, they also have to be integrated into more complex models of the nursery as a whole to address planning, scheduling, and logistics. For policy makers, there is a need for models at regional or national scale that help them to decide on measures related to environmental issues or economic development. Models on product quality and integration of models from different disciplines to simulate nurseries and whole product chains are some of the important and challenging developments in greenhouse simulation over the last few years. Also, the implementation of models into practice is a hot issue generating many new and complex research questions.