Algae, both microalgae and seaweeds, has received considerable attention recently on a global scale as a potential source of valuable compounds with applications in agri/food, health/pharma, and cosmetics industries. Both primary and secondary algal metabolites have demonstrated a wide range of biological activities with multiple potential uses. However, before the full potential of algal bioactives can be realized, there remain some outstanding knowledge gaps regarding characterisation, activity and applicability of algal compounds. These include the natural chemical complexity, variability, and plasticity of algal compounds underpinned by a vast systematic diversity and environment-induced modifications which are, to-date, only partially understood. Many biological activities are also reported only in vitro, and potential interactive effects, bioavailability of bioactive compounds in the desired application, as well as stability during processing require further attention. Additionally, the sustainable supply of traceable biomass with a known, stable, and reproducible chemical composition remains a challenge.