Cyanobacteria metabolism is based on oxygenic photosynthesis, which makes them interesting organisms for constructing of green cell factories. Autotrophic growth requires a tight adjustment of carbon and nitrogen assimilatory reactions, which relies on a precise sensing of central metabolism. Hereby the ubiquitous PII signalling protein plays a kay role in maintaining C/N homeostasis. Under unbalanced nutrient supply, reserve polymers such as glycogen, polyhydroxybutyrate or cyanophycin are accumulated, processes, in which PII regulation is intimately involved. An important control point of the carbon flux is the 2,3-bisphosphoglycerate-independent phosphoglycerate-mutase (PGAM), which converts the first CO2 fixation product 3-phosphoglycerate to 2-phosphoglycerate, directing carbon flow towards lower glycolysis for the production of amino acids, fatty acids and biopolymers.(...) present our current metabolic engineering strategies to maximize biopolymer synthesis in cyanobacteria.