In the Netherlands about 2000 ha of glasshouses is equipped with supplementary assimilation light (SL), which is about 19% of the total glasshouse area. Besides increased production, SL results in improved product quality, a better control of yield and quality, possibilities for earlier or year-round production and a more regular labor requirement. In this paper several recent experiments with different strategies of SL (33 up to 210 µmol m -2 s -1 ) for tomato, sweet pepper, cucumber and eggplant are presented and discussed. In general, it was concluded that SL was not economically feasible. For cucumber SL can only be attractive if the crop is grown at high plant density and according to the high-wire system. Based on 3 plantings per year, a production of 147 kg m -2 (360 cucumbers) is possible with 210 µmol m -2 s -1 SL during 3000 h/year. Application of 50% of the light within the crop (interlighting) by fluorescent tubes instead of only HPS-lamps above the crop, did not improve production but improved fruit quality in cucumber. Mobile lamps are sometimes used instead of fixed lamps. For sweet pepper and tomato, a fixed-lamp installation was economically more feasible than mobile lamps when compared at the same light intensity. A dynamic simulation model was used to predict effects of different lighting strategies at 500 and 1000 ppm CO 2 on potential production. Maximum levels of 110, 64 and 168 kg m -2 year -1 were calculated for tomato, sweet pepper and cucumber, respectively. INTRODUCTIONThe glasshouse industry in the Netherlands is the largest worldwide with 10.500 ha and a production value of 5.9 billion euro. About 40% of the glasshouse area is used for vegetable production, mainly tomato, sweet pepper and cucumber. Ornamentals, both cut flowers and pot plants are grown on the remaining 60%. Main cut flowers are roses and chrysanthemum, whereas ficus, kalanchoe and begonia are the most important pot plants considering cultivation area. Although the total glasshouse area is rather constant, the area of individual companies is growing fast. For vegetables, in 1995 442 companies (9% of the total) were larger than 2 ha, whereas in 2004 this was 686 (26% of the total). Among these are several companies larger than 20 ha. Annual production levels in the Netherlands belong to the highest in the world, e.g. for tomato (up to 70 kg m -2 ), cucumber (up to 90 kg m -2 ), large-sized cut roses under supplementary assimilation light (SL; 270 stems m -2 ) and cut chrysanthemum under SL (250 stems m -2 ). A major threat to the glasshouse industry is its high energy use. The equivalent of 4.3 × 10 9 m 3 of natural gas is used annually to heat the glasshouses, resulting in an average gas input of 41 m