Inhibition of photosynthesis by excess excitation energy is initiated in the reaction center of photosystem II. The primary site of photoinhibition in the reaction center (components of primary charge separation or secondary electron acceptor QB) is still disputed. Photoinhibition is characterized by quenching of variable chlorophyll flurescence (Fv), resulting from increased thermal dissipation of excitation energy. Varying responses of initial fluorescence (F0), however, seem to indicate involvement of different mechanisms. As far as photoinhibition is reversible within minutes to hours, it can be viewed as a controlled protective mechanism that serves to dissipate excessive energy, Supposedly, another dissipative mechanism, distinguished by its faster kinetics (response within seconds), is related to the energy-dependent fluorescence quenching.