Algae include Seaweeds and Microalgae. These largely underutilised resources are rich in proteins and fibre and could be considered as future feeds due to their abundance and ability to be produced without land and to a degree without freshwater. Traditionally, seaweeds were consumed as fodder in times of climatic hardship, in areas spanning from Scandinavia to the Mediterranean. Today, they are close to being considered as viable protein sources for feed use. Their development in the coming years depends on their production using aquaculture, as well as scalable technologies to harness the nutrient fractions in a circular manner and utilising independently anti-nutritional factors as feed additives. Antinutritional factors include phlorotannins, saponins and other small molecules that show promise for use as agents to reduce methane and GHG emissions, provide anthelmintic function and immunity when included as additives in feeds for ruminants, monogastric animals and companion animals.