In relation to the control of crop growth it is necessary to consider the aspect of time constants. Some growth characteristics, such as photosynthesis and transpiration have a rapid response to control actions, whereas other characteristics, such as development and morphogenesis show a much slower response. It is therefore important to distinguish different levels of control, according to the response time involved. For each level of control different crop growth models are to be used, with different requirements. In climate control one is primarily concerned with rapid responses of the crop. Crop growth models used for climate control therefore should be particularly strong in an accurate prediction of the instantaneous rate of photosynthesis and transpiration of the crop in relation to the environmental factors. Models that are used for prediction of crop growth over weeks or months can deal with photosynthesis in a more simple way. There are, however, much higher demands with respect to developmental processes such as formation of leaves, flowers, fruits, maturing, etc. Models used for this purpose are more crop specific than the first mentioned category and due to a lack of basic understanding of the underlying processes, have to be more descriptive.