Long time considered as secondary molecules, the vital importance of carbohydrates is now re-evaluated. This dramatic change is mainly induced by the recent findings proving glycoconjugate role in cell recognition and adhesion, basic processes involving homotypic and/or heterotypic interactions. Our initial investigations in marine invertebrate provided direct evidence that primordial proteoglycans can also mediate these cellular processes via a new alternative molecular mechanism: polyvalent glycan-glycan binding. The proteoglycans, beside mucins, are the largest macromolecules extending above the cell surface. As the most peripheral cell surface environment sensors, the proteoglycans may have provided the first key recognition and adhesion functions during the emergence of multicellular forms of life. If so, the simplest Metazoans alive today, such as Porifera, should have preserved, at least in part, proteoglycan recognition and adhesion mechanisms guiding the beginning phase of xenogenic selectivity of cellular interactions. Using atomic force microscopy, we have demonstrated that proteoglycan-proteoglycan and calcium ions are essential for cell adhesion in the marine sponge Microciona prolifera. The large cohesive force between two individual sponge proteoglycans is able to hold the weight of approximately 1,600 cells, and thus to preserve the integrity of the sponge organism. These results provided the first essential and quantitative evidence that carbohydrate-carbohydrate binding can perform the adhesion function assigned to it. Such cell adhesion proteoglycan molecules, named by us glyconectins, underlie the molecular mechanism of self-recognition and adhesion. The main aim of our projects is to decipher the molecular mechanisms of glycan-glycan interactions involved in cell recognition and adhesion processes.