Marine macroalgae biomass is often used for the production of ingredients in cosmetics. These ingredients can have one of the three main functions: (1) they are considered as additives which contribute to the organoleptic properties; (2) they are used for stabilisation and preservation of the product; (3) or finally, they are bioactive compounds which fulfil a real cosmetic function and activity. This chapter presents the bioactivities of molecules or extracts used for cosmetic applications, and discusses some perspectives for the development of new compounds and specific extraction methods. Metabolites derived from seaweeds have been shown to be active in antiaging skin care, anticellulite treatment and slimming, as well as having antioxidant, photoprotective, moisturising, and whitening properties. Among the various classes of seaweed components, sulphated polysaccharides, peptides, carotenoids, fatty acids, and phytohormones exhibit antiaging and antioxidant properties, while mycosporine-like amino acids, flavonoids have an antiphotoaging activity. Flavonoids (i.e. phlorotannins) are lipolytic agents which are isolated from macroalgae and which also inhibit melanogenesis. A better knowledge of algae genetics and the improvement of algae cultivation or co-cultivation will provide new opportunities for the development of bioactive compounds.