The phenomenon of photoinhibitíon, by which all organisms performing oxygenic photosynthesis decrease their photosynthetic activity when exposed to excessive irradiance, depends, at molecular level, mainly on light-induced modifícations of photosystem 2 (PS2). In the last decade a great deal of research has been focused on the molecular mechanisms responsible for these modifícations and some of the processes involved háve been clarified. Inactivation of PS2 and degradation of its components, as well as some of the mechanisms involved in the restoration of PS2 activity can be studied in vitro using preparations characterized by various degree of complexity from entire thylakoid membrane to the simplest preparation of isolated PS2 reactíon centre. Although the in vitro approach contributes essentíal informatíon to our understasiUng of the molecular process underlying photoinhibitíon, extrapolatíon to the in vivo situation is not obvious and many points demand íiirther experimental work.