The phenology of polar seaweeds is strongly tuned to the strong seasonal changes in underwater radiation. Daylength triggers the onset of reproduction and growth during winter in season anticipators, particularly in endemic species. More widely distributed species are often season responders growing predominantly in summer in direct response to light, temperature and nutrient conditions. The physiology of polar seaweeds is strongly linked to the life strategy of the individual species. For season anticipators, photosynthetic rates are often highest in late winter-spring when sunlight penetrates deep into the clear waters. Since Antarctic species are seldom nutrient-limited, even during summer, they mostly incorporate photosynthetically fixed carbon directly into biomass. In contrast, Arctic kelps, such as Laminaria solidungula utilise the carbon fixed during summer for synthesis of storage compounds, which are used to fuel growth during the dark winter period when nutrients are sufficient for new tissue formation. This growth pattern reflects a strategy for species optimally adapted to the seasonal changes of nutrient concentrations, which in the Arctic are low (or nearly undetectable) in summer, and high in winter. This review concludes with a discussion of possible implications of global climate changes on the phenology and productivity of polar seaweed communities.