The use of lamps for improvement of CO2-assimilation of greenhouse crops has increased enormously during the last 15 years in The Netherlands. The main reasons for the use of assimilation lighting are not only to increase crop production and product quality, especially in winter time, but also more and more to ensure a year round production and quality level which meets market demands. Assimilation lighting is often used for plant propagation and rose production, while its use in other ornamental crops is increasing rapidly. In addition, few producers of vegetables started to apply assimilation lighting. Some new trends are higher light intensities during longer periods, techniques to improve light output per lamp by enlarging the voltage and moveable lamps with high light intensities. The coming years the use of artificial lighting in the Netherlands will be affected by liberalization and legislation of the trade of energy (gas and electricity). Nowadays explanatory crop models are available that can predict crop growth quite accurately. These models in combination with models simulating the greenhouse climate are powerful tools to analyse effects of artificial lighting. Some results of a combined model for crop growth and greenhouse climate are shown with respect to different strategies for the operation of artificial lighting. It is shown that the light use efficiency (unit production per unit of PAR from lamps) decreases when intensive lighting regimes are applied. Besides light, lamps produce heat. Therefore, the energy demand of the heating system decreases when lights are turned on. This simulation study showed that it is possible to increase the energy use efficiency (unit production per unit of energy input from lamps and heating system together) by the use of lighting.