Photosynthetic organisms constantly face the threat of photo-oxidative stress from fluctuating light conditions and environmental stress. Plants and algae have developed an array of defences to protect the chloroplast from reactive oxygen species. Genetic and physiological studies have shown that antioxidant responses are important to high-light acclimation, both by directly scavenging or quenching reactive oxygen intermediates and by contributing reducing power for alternative electron transport pathways and excess energy dissipation. At present, the signalling events leading to up-regulation of antioxidant defences in high light remain a mystery. Recent advances toward understanding acclimation to oxidative stress in both photosynthetic and non-photosynthetic model organisms may illuminate how plants and algae respond to high-light stress. Although the role of hydrogen peroxide in high-light acclimation has been investigated, less is known about responses to singlet oxygen, a form of reactive oxygen that poses a significant threat specifically to photosynthetic organisms. This review will discuss some intriguing new findings in that area, focusing on recent findings regarding the nature of singlet-oxygen responses in the chloroplast.