Marine macrophytes live in a high variable environment in respect to light, UV-radiation, temperature and also salinity. During evolution their metabolism has adapted to these strongly changing conditions, especially occurring in the upper littoral zone. In the lower subtidal, the photosynthetic apparatus has adapted to low light conditions, to absorb maximally the inci-dent photons and to utilize the absorbed energy with high efficiency. In the intertidal and up-per sublittoral zone the photosynthetic apparatus is exposed to strong light stress when the ab-sorbed amount of light energy is higher than the organism can use for its metabolism. This happens generally when plants are expo-sed to irradiances, which are above the mean level of irradia-tion occurring at their growth sites (e.g., light flecks or low tide around noon). Under light stress the amount of thermal energy dissipa-tion increases and the energy flowing into photo-chemistry decreases also. Photoin-hi-bition causes a decrease of the photosyn-thetic quantum yield (dynamic inhibition) and at much higher fluence rates or after a longer duration of high irradiation, also a de-crease of the pho-tosyn-thetic capacity (chronic inhibition). The absorp-tion cross-section of the chroma-topho-res can be diminished by chlo-ro-plast displacement or shrin-king, especially in brown algae, which decreases the rate of photodamage to the light-ab-sorbing apparatus. Irra-dian-ces, which exceed the protective ca-pacities of these mecha-nisms, cause irreversible damage to the photo-syn-thetic appara-tus. Normally, the latter does not occur at the natural growth sites.In the daily cycle, dynamic photoinhibition of photosynthesis generally occurs in intertidal and upper subtidal algae, especially if low tide coincides with noon. Dynamic photoinhibition is a protective me-chanism rather than a photo-damage. It enables the photo-syn-thetic appa-ratus of macroalgae to cope with the temporary light stress in the upper littoral zones. Algae which are not able to decrease the energy pressure on photosyn-thesis regulative by harmless thermal en-ergy dissipation and/or a decrease of the absorption cross section (see Chapter 16) are irreversibly damaged and, therefore, cannot grow in the upper littoral zones.Kirk (1994) stated in his textbook that many macrophytes show photo-in-hibition of photo-synthesis. However, he concluded misleadingly that "ecologi-cally this pheno-menon is of less signifi-cance, since any given macrophyte spe-cies is generally to be found growing at a depth where the light intensity is one to which it is well adapted". On the contrary, the studies performed in recent years show clearly that photoin-hibition of photosynthesis is of great signifi-cance for the survival of macro-phytes in their natural environ-ment. In particular, algal zonation is thought to de-pend at least in part on the ability for photoin-hi-bition.This chapter deals, therefore, with the special environmental conditions for marine macro-phytes, the adaptation and acclimation of photosynthesis to these conditions, and it discusses fluorescence techniques recently developed especially for in situ (field) measurements.