With today's interest in novel renewable chemicals and polymers, the underexploited marine green algae belonging to species of Ulva and Entermorpha stimulated interest as sources of polysaccharides with innovative structure and functional properties. These algae are common on all seashores and can produce in time an important amount of biomass in nutrient-enriched waters. The major water-soluble polysaccharide, ulvan, extracted from the cell wall represents about 8-29% of the algae dry weight. The original physicochemical, rheological, and biological properties recently unraveled for this complex sulfated aldobiouronan open the way for novel potential applications.