Photoprotection is the biochemical process that helps organisms cope with molecular damage caused by sunlight. Plants and other oxygenic phototrophs have developed a suite of photoprotective mechanisms to prevent photoinhibition and oxidative stress caused by excess or fluctuating light conditions. Humans and other animals have also developed photoprotective mechanisms to avoid UV photodamage to the skin, prevent DNA damage, and minimize the downstream effects of oxidative stress.

In organisms that perform oxygenic photosynthesis, excess light may lead to photoinhibition, or photoinactivation of the reaction centers, a process that does not necessarily involve chemical damage. When photosynthetic antenna pigments such as chlorophyll are excited by light absorption, unproductive reactions may occur by charge transfer to molecules with unpaired electrons. Because oxygenic phototrophs generate O₂ as a byproduct from the photocatalyzed splitting of water (H₂O), photosynthetic organisms have a particular risk of forming reactive oxygen species.^{[citation needed]}

Therefore, a diverse suite of mechanisms has developed in photosynthetic organisms to mitigate these potential threats, which become exacerbated under high irradiance, fluctuating light conditions, in adverse environmental conditions such as cold or drought, and while experiencing nutrient deficiencies which cause an imbalance between energetic sinks and sources.

In eukaryotic phototrophs, these mechanisms include non-photochemical quenching mechanisms such as the xanthophyll cycle, biochemical pathways which serve as "relief valves", structural rearrangements of the complexes in the photosynthetic apparatus, and use of antioxidant molecules. Higher plants sometimes employ strategies such as reorientation of leaf axes to minimize incident light striking the surface. Mechanisms may also act on a longer time-scale, such as up-regulation of stress response proteins or down-regulation of pigment biosynthesis, although these processes are better characterized as "photoacclimatization" processes.

Cyanobacteria possess some unique strategies for photoprotection which have not been identified in plants nor in algae. For example, most cyanobacteria possess an Orange Carotenoid Protein (OCP), which serves as a novel form of non-photochemical quenching. Another unique, albeit poorly-understood, cyanobacterial strategy involves the IsiA chlorophyll-binding protein, which can aggregate with carotenoids and form rings around the PSI reaction center complexes to aid in photoprotective energy dissipation. Some other cyanobacterial strategies may involve state-transitions of the phycobilisome antenna complex , photoreduction of water with the Flavodiiron proteins, and futile cycling of CO₂ .

It is widely known that plants need light to survive, grow and reproduce. It is often assumed that more light is always beneficial; however, excess light can actually be harmful for some species of plants. Just as animals require a fine balance of resources, plants require a specific balance of light intensity and wavelength for optimal growth (this can vary from plant to plant). Optimizing the process of photosynthesis is essential for survival when environmental conditions are ideal and acclimation when environmental conditions are severe. When exposed to high light intensity, a plant reacts to mitigate the harmful effects of excess light.

Plants are delicately able to obtain sufficient solar light for photosynthesis while preventing the damage it could cause. While chlorophyll is very efficient in absorbing visible light for photosynthesis, no photosynthetic organism can use all this energy in full sunlight. Moreover, stresses in the environment, such as extreme temperatures in winter and summer, high salinity, and low moisture or nutrients, also decrease photosynthesis.

Plant photoprotection has three main levels. One is the reflection or screening of light. This may involve leaf senescence in stress conditions. Second is conversion of light energy into heat. As a powerful method, ranging from seconds to seasons, it is used only in excessive light conditions, maximizing absorbed light for photosynthesis. Similarly, infrared radiation from the sun is given as heat from heating water in plant cells; it is not a hazard except in extreme heat or when unable to evaporatively cool itself. Therefore the principal threat to leaves is found in visible light. Thirdly, and most importantly, is detoxification of toxic radicals from too much light. If light is not channeled into photochemistry, it can leave behind excessive energy from either chlorophyll or the electron transport chain. This can lead to reactive oxygen species, which can damage molecules such as DNA, proteins, and lipids in cell membranes, or even cell death. Such effects are even more severe when there are stressors. The reactive oxygen species are detoxified by antioxidant enzymes and metabolites (e.g., vitamins C and E) prior to reaction with other molecules. While this anti-oxidation method is ubiquitous among plants, it is most common when light changes temporarily override heat dissipation. When antioxidant enzymes are inhibited, as in low-temperature winter stress conditions, so is detoxification of oxygen. Heat must therefore be generated to decrease light levels when excessive, but not when sufficient, which would interfere with photosynthesis; these processes are exquisitely coordinated.