In nature, plants are permanently exposed to adverse environmental factors. Chlorophyll a fluorescence is one of the most highly informative, rapid and non-destructive diagnostic methods for the detection and quantification of damage in the photosynthetic apparatus caused by environmental stress. Chlorophyll a fluorescence is used to assess PSII status under exposure to light stress, cooling, high temperature, and water stress. This chapter presents specific examples of chlorophyll fluorescence applications in different stress situations. In plant physiological research, including studies of drought stress, one of the most frequently used fluorescence parameters is the maximum quantum yield of PSII photochemistry (F_V/F_M). This parameter represents a generally acknowledged measure of photosynthetic status; it is very easy to measure, and its interpretation seems to be very simple. When light intensity is high, some of the energy absorbed by photosynthetic pigments cannot be used for photosynthesis.