This chapter discusses the effect of anionic polysaccharides extracted from brown and red algae, fucans and galactans, on angiogenesis. The anionic polysaccharides from the brown seaweed Sargassum stenophyllium, which is composed of L-fucose, manose, galactose, xylose, glucuronic acid and sulfate, altered the vessel formation in the vascular membrane of a chick yolk sac. Marine organisms such as brown and red seaweeds are rich sources of sulfated polysaccharides fucans or fucoidans and galactans, respectively. These polysaccharides are structural components of the extra-cellular matrix. Fraction rich in fucans with 34.1% of sulfate obtained from Lobophora variegata showed antiangiogenic action in chorioallantoic membrane assay. Anionic polysaccharide of low molecular weight from brown seaweed shows proangiogenic activity. The red seaweed Amansia multifida is a source of sulfated galactans composed of minor quantities of glucose, xylose, mannose and glucuronic acid. The fraction (FT) with antiangiogenic activity. FT, total factor or crude polysaccharides presented antiangiogenic activity in all tested concentrations.