Differences in morphology, size, and pigmentation among various phytoplankton taxonomic groups impact their light absorption and scattering properties (e.g., Morel and Bricaud, 1981; Stramski and Kiefer, 1991; IOCCG, 2014), which modifies the color of the ocean. Optical satellite remote sensing enables the detection of backscattered sunlight emanating from the water surface (so-called ocean color). These observations can be exploited to obtain information not only on the overall biomass of phytoplankton, but also on the absence, presence, and dominance, of different phytoplankton groups. The remote-sensing products provide synoptic coverage of surface waters at global scale, and with a spatial coverage impossible from in-situ sampling. In this chapter, we define phytoplankton groups (PG) based on taxonomic criteria and also categorize phytoplankton composition using three phytoplankton size classes (PSC), following the classification of Sieburth et al. (1978). We discuss the characteristics required of satellite ocean-color sensors for PG and PSC detection, the algorithms' principles, we show applications of these satellite products, and discuss the societal impacts of these data sets.