During recent years there has been remarkable progress in the understanding and practical use of chlorophyll fluorescence in plant science. This 'renaissance' of chlorophyll fluorescence was induced by the urgent need of applied research (like plant stress physiology, ecophysiology, phytopathology etc.) for quantitative, non-invasive, rapid methods to assess photosynthesis in intact leaves. Recent developments of suitable instrumentation and methodology have substantially increased these possibilities. Actually, a vast amount of knowledge on chlorophyll fluorescence had already accumulated over more than 50 years, since the discovery of the Kautsky effect in 1931 (Kautsky and Hirsch 1931) (for reviews, see e.g., Lavorel and Etienne 1977, Briantais et al. 1986, Renger and Schreiber 1986). On the one hand this knowledge was mechanistic, resulting from biophysically oriented basic research. On the other hand it was phenomenological, originating from applied plant physiological research. Until recently the phenomenology of whole leaf chlorophyll fluorescence appeared far too complex to find serious attention of biophysicists. Thus, for a long time, there was a gap between applied and basic research in chlorophyll fluorescence. Developments in instrumentation (Ogren and Baker 1985, Schreiber 1986, Schreiber et al. 1986) and methodology (Bradbury and Baker 1981, Krause et al. 1982, Quick and Horton 1984, Dietz et al. 1985, Demmig et al. 1987, Weis and Berry 1987, Bilger et al. 1989, Genty et al. 1989) has succeeded in closing this gap and bringing these two disciplines into sufficiently close contact and in mutually stimulating interaction. Consequently the present "renaissance" of chlorophyll fluorescence may be the product of a fruitful dynamic interaction between three different research disciplines, i.e., basic and applied research linked to new developments in instrumentation and methodology (see scheme in Fig. 1). As a result, measuring chlorophyll fluorescence has become a very attractive means of obtaining rapid, semiquantitative information on photosynthesis, used by an increasing number of researchers not only in the laboratory but also in the field. The wide range of possible applications is reflected by the broad spectrum of contributions to this issue of Photosynthesis Research. The progress made in chlorophyll fluorescence instrumentation and methodology has also induced new developments in the adjacent fields of absorbance spectroscopy (e.g., Klughammer et al. or Harbinson et al. in this issue), photoacoustic spectroscopy (e.g., Canaani, Dau and Hansen, Kolbowski et al. or Snel et al. in this issue) and chlorophyll luminescence (delayed fluorescence) (Bilger and Schreiber in this issue). These new developments are expected to play a role in