Marine organisms are considered an excellent source of bioactive compounds; within these options, microalgae have been of significant interest in recent years. Microalgae are rich in nutrients and biologically active substances such as proteins, polysaccharides, lipids, polyunsaturated fatty acids (PUFAs), vitamins, pigments, enzymes, etc. These bioactive substances are demanded in different areas such as pharmacology, cosmetics, medicine, the chemical industry, and agriculture, among others, because they can present various effects such as antioxidant, immunostimulant, neuroprotective, antitumor, antihypertensive, anti-viral, anti-bacterial, and regenerative, among others. This chapter refers to the bioactive compounds produced by microalgae, such as polyphenols, carotenoids, fatty acids, terpenoids, polysaccharides, and peptides, as well as the biological activities attributed to them, such as antioxidants, anti-inflammatory, anticarcinogenic, and antimicrobial, among others.