Polyhydroxyalkanoates (PHAs) are a group of naturally occurring polymers produced by microorganisms, among which poly (3-hydroxybutyrate) (PHB) is the most studied biodegradable polymer that accumulates in bacteria as carbon reserve in the form of inclusion bodies when cells grow under stress conditions. Current industrial PHB production processes rely mostly on the availability of agricultural resources, which are costly and unsustainable having sometimes an ecological footprint. Besides sustainability, the PHB production in cyanobacteria using CO2 and sunlight has the Advantage of reducing the production cost of this biodegradable polymer. Nevertheless, the PHB production in cyanobacteria from an economic point of view has two major drawbacks: slow cellular growth and little productivity. In order to promote photosynthetic PHB production on an industrial scale, the productivity needs significant improvement and industrially relevant cyanobacterial strains need to be optimized. Recent research has mainly focused on genetic engineering to increase PHB productivity, which mainly reports as a higher percentage of dry cell weight content. The studies have rarely reported an increase in photosynthetic Efficiency or an increase in the specific growth rates or production rates. With respect to commercialization and scale-up of the cyanobacterial PHB production, especially with the regard to EU-legislations on the petrochemical plastics, a ‘holistic approach, considering the view of the whole process, is required. A proper, quantitative time-resolved analysis of the PHB production mechanism and the physiological adaptations to media limitations may help to increase photosynthetic PHB productivity. On the basis of the mentioned hypothesis, the aim of the work described in this thesis was to explore the PHB production mechanism in cyanobacteria, using the knowledge, to improve the photosynthetic polymer production processes for industrial applications. This is needed to make cyanobacterial polymer production economically feasible and competitive with synthetic polymers and other biodegradable plastics in the market. Within this thesis, a systematic, quantitative, time-resolved analysis of the process to prove economic feasibility of photosynthetic polymer production is presented, which is divided into three main tasks: i) Characterization of the process using a quantitative, analytical Approach to guarantee a reproducible bioprocess ii) The development of industrially relevant strains with superior characteristics and iii) Understanding the metabolic responses to nutrient Limitation and the influence on PHB formation to optimize the production process. The thesis shows: A review of the state of the art and challenges associated with cyanobacterial Bioprocess engineering. Unexplored potential of wild-type cyanobacteria to increase phototrophic PHB productivity, as an example the unicellular strain Synechocystis sp. PCC 6714 is presented as photosynthetic PHB producer. Quantitative, time-resolved characterization of the process facilitates bioprocess optimization and control. The approach significantly increased the polymer productivity in Synechocystis sp. PCC 6714. Random mutagenesis shows huge potential for the development of industrially relevant strains for PHB production and can help identify target genes for future genetic engineering. The selected high PHB yielding mutant, MT_a24 was generated by UV-mutagenesis showing exceptional fitness. Understanding the physiological responses to media limitation is essential to optimize and scale production. An easily scalable, one-step process could be established with increased PHB productivity in preference to the commonly done two-step processes. A bioprocess with higher productivity and economic feasibility could be developed within this thesis. The thesis contributes towards a better understanding and set-up of an adequate process for phototrophic PHB production. The results of this work can be used to develop strategies to enhance productivity in other microalgae and cyanobacterial strains and to generate strains suitable for large-scale, outdoor cultivations.