This chapter discusses the situation of local algal-biomass production and its applications as well as the high-value biochemical compounds present in micro- and macroalgae, including their extractions and potential uses. Microalgae and macroalgae are well recognized in the food market and in agricultural and industrial sectors. Micro- and macroalgae are found in many parts of Thailand due to favourable climatic conditions. The chapter explains the locally grown strains of marine- and freshwater algae and their applications. In Thailand, freshwater macroalgae is much less popular than marine macroalgae. Marine and freshwater macroalgae are natural sources of carotenoids, polysaccharides and phenolic compounds, which exhibit antioxidant, antigastric ulcer, anti-inflammatory, analgesic and hypotensive activities, including anticancer and antimicrobial activities. In Thailand, microalgae are well known for two applications, namely, food supplements and feed additives for aquaculture. The trend in algal biotechnology research in Thailand has mainly shifted to molecular biological studies.