Phlorotannins are the most abundant group of metabolites specific for brown algae. These substances contribute both to the primary and secondary metabolism of the algal cells and have practical relevance as biologically active compounds. The list of their presumable physiological functions is still not exhaustive and includes wound healing, chelation of heavy metal ions, bioadhesion, contribution to the processes of algal early embryogenesis and sporogenesis, etc. Similar to higher plant phenolics, phlorotannins also have antioxidant properties, provide chemical defense against herbivores and contribute to cell wall rigidification. The complex and diverse composition of natural phlorotannins hampers investigation of their physiological roles and leads to inconsistencies in the obtained data. Further study of the correlation between the structure of these substances and their functions is needed to take a new look at known information, thus providing better performance in the fields of both fundamental algal physiology and applied phycology.