Some personal comments are given about research on supplementary lighting in horticulture. There is a rapid development in the use of supplementary lighting in horticultural practice. Generalisation is needed to share, transfer and apply the pertinent knowledge. Knowledge at process level is therefore indispensable. The distinction between photosynthesis enhancement and control functions of light should be considered explicitly. Especially in applied research experimental details are often insufficiently reported and experiments tend to follow too much a trial and error approach. An integrated problem analysis, taking system levels and scale problems into account, would contribute to efficiency in research. Rules for maximum day length for supplementary lighting, the role of the spectrum of HPS light and a more systematic approach in research on lighting for plant control are subjects that should be resolved in the near future.