This chapter reviews the nature of pigment variations in phytoplankton in response to changes in light regime (irradiance, spectral composition and daylength). These changes belonging to processes called acclimation and/or adaptation maximize the evolutionary fitness of a species, within the constraints set by the environmental conditions (Raven and Geider, 2003). In general, adaptation indicates long-term evolutionary outcome based on the genes a given species obtains (genetic adaptation) while acclimation denotes adjustments in response to variation in key-environmental variables (physiological acclimation). Photo-acclimation corresponds to a mosaic of processes involving many cellular components and occurring over a broad range of time scales, from seconds to days. These processes, covering many physiological, biochemical, biophysical and biological changes, allow the optimization of cell activities, such as photosynthesis, respiration, growth and division when faced with changing irradiance (e.g., Herzig and Dubinsky, 1993; Anning et al., 2000; Raven and Geider, 2003). This is an important issue in phytoplankton ecology because of the fluctuating light environment experienced by pelagic algae, related to daylight variations together with the exponential decrease of light and the vertical – active or passive – movements of algae along the water column. In order to cope with these never-ending fluctuations in light-regime, marine phytoplankton can adjust their pigment pool, which is mainly constituted by two functional categories, namely pigments used for light harvesting and for photoprotection. Many accessory pigments constituting the light-harvesting complexes are photosynthetically active i.e. they are able to transfer the energy absorbed from sunlight to the photosynthetic reaction centers (RC) of photosystems (PS) II and I. They are called light harvesting pigments and include the photosynthetic carotenoids. However, some carotenoids are not involved in photosynthesis and do not transfer the absorbed energy to the RC. These non-photosynthetically active carotenoids are also called photoprotective carotenoids (PPC). The function and dynamics of long-term (hours-days) and short-term (minutes-hours) photo-acclimation are described in the following two sections (4.4.1 and 4.4.2, respectively). The long-term photo-acclimation response mainly consists in changes of structure and composition of the photosystems while the short-term photo-acclimation process mainly concerns the xanthophyll cycle (XC) activation and the associated non-photochemical fluorescence quenching (NPQ). In the third section (4.4.3), the ecophysiological variability of XC and its use as a biological tracer in oceanographic studies is reported.