Many advances have been made in recent years regarding the use of algae as food and medicine. Seaweed in Health and Disease Prevention, Second Edition, aims to update knowledge on the therapeutic and functional activities related to algae and their compounds. This includes a new chapter on the use of seaweed and seaweed products in the current war against viral-based exposures, a result of the massive research effort stemming from the global Covid Pandemic, along with the improvement of the nutritional value of algae via enzymatic or fermentation treatments. These treatments are at the origin of the production of oligosaccharides presenting interesting biological activities for the human microbiota. Chapter updates on seaweed’s biochemical characteristics such as proteins, lipids, and polysaccharides will also be addressed.