The flexible macro-organisation of the photosynthetic thylakoid membrane allows plants to respond to changes in environmental conditions. Under excess light conditions light harvesting antenna complexes (LHCII) switch to a protective state, in which excess energy is dissipated as heat - a process called non-photochemical quenching (NPQ). The rapidly reversible component ofNPQ (qE) is dependent on the acidification of the thylakoid lumen and is regulated by the xanthophyll cycle. The PsbS protein plays pivotal role in this process, but its mode ofaction is unknown. This thesis presents an investigation of the effects of PsbS on NPQ,particularly exploring its function in the organisation ofthe antenna complexes. Mutant plants lacking or over-expressing PsbS were used in the study ofNPQ. PsbS was demonstrated not to be necessary for the formation of qE, since even in the absence of the protein there was a small amount of reversible quenching that was ~pH-dependent arid sensitive to inhibition ofthe xanthophyll cycle. The level ofPsbS was shown to affect both the extent and the kinetics of the formation of qE. The temperature sensitivity of qE was also shown to be controlled by PsbS. These data suggest that PsbS is a catalyst ofquenching formation. PsbS was shown to function in the macro-organisation of the thylakoid membranes. The protein affected the macro-structure and the detergent sensitivity of the membranes. In unstacked thylakoids the Mi+-induced re-assembly of the PSIILHCII supercomplexes was shown to be highly dependent on PsbS. By comparing various types of mutants, it was shown that this effect of PsbS depended on the level of expressed protein and not whether it was active in qE formation. It is argued that the roles of PsbS in the Ipacro-structure of the thylakoid membrane and in qE formation both reflect the LHCII-organising properties ofthe protein.