Stress is considered to be major problem for plants, wether they are in cultivation or growing in natural settings. Consequently there is a need to have some means of identifying stress. Recently developed techniques for measuring photosynthetic processes using non-destructive biophysical techniques, especially chlorophyll fluorescence, offer a powerful and convenient tool for probing the physiological state of the plant. They have also been exploited for the detection of stress, screening for stress tolerance, and for quality determination. The physiological basis, techniques and application of these techniques will be discussed. The problems associated with definition of stress will also be considered.