## 1 Introduction  
Increasing concerns over energy and environmental problems prompted the need to develop renewable chemicals and fuels. Advances in genetic manipulation and genomics understanding enabled rapid advancement of microbial cell factory development. Once the enzymatic and genetic information for metabolic pathways producing important commodity biochemicals are solved, these genetic parts are then transferred to other organisms capable of utilizing diverse bioresources for growth. Among the various microbial systems, cyanobacteria have received an enormous amount of attention in recent years. Cyanobacteria are photosynthetic prokaryotes that grow through the direct utilization of sunlight and CO\({}_{2}\). Cyanobacteria generate ATP and NADPH through oxygenic photosynthesis, and subsequently use them to fix CO\({}_{2}\) into central metabolite through the Calvin-Benson-Bassham (CBB) cycle (Figure 1). By tapping into the metabolism of cyanobacteria, central metabolites may be directly transformed into desired biochemicals. This approach bypasses the need of repeated construction and deconstruction of plant biomass, which potentially increases the overall solar energy conversion efficiency. Most commonly used cyanobacterial models (Table 1) are _Synechococcus elongatus_ PCC 7942, _Synechocystis_ PCC 6803, _Synechococcus_ sp. PCC 7002, and _Anabaena_ sp. PCC 7120 (hereafter referred to as PCC 7942, PCC 6803, PCC 7002, and PCC 7120, respectively). The genomes of these model cyanobacteria have been sequenced. In addition, these sequences have been organized and compiled into databases, such as cyanobase [1], for rapid access. The construction of these genome databases enabled the interpretation of cyanobacterial metabolism and facilitated the development of recombinant engineering. In addition, metabolomics analyses on these cyanobacteria have been reported [2; 3; 4]. This information, together with the construction of metabolic models [5; 6; 7], is useful for both understanding the basic metabolism of cyanobacteria and achieving higher level of metabolic redirection and control. PCC 7942 and PCC 6803 are freshwater cyanobacteria, while PCC 7002 is a marine species with the additional advantage of having higher salt tolerance. PCC 7120 is diazotrophic, which means it can utilize N\({}_{2}\) as nitrogen source. With the exception of PCC 7120, all other model strains are naturally competent [8; 9]. PCC 7120 transformation have been carried out through conjugation [10]. Cyanobacterial carbon and energy metabolisms differ significantly from that of heterotrophs commonly used in fermentation. As a result, additional metabolic engineering strategies such as changing reducing cofactor preference from NADH to NADPH, expressing pathways with higher thermodynamic driving force, and expressing transporters for excretion of products are often needed. Here we summarize the advances made in the metabolic engineering of cyanobacteria for the production of biochemicals and discuss the strategies and pathways used to improve their productivities. Table 2 is a comprehensive list containing biochemical targets, genes and promoters used, and production titers. The relevant metabolites associated with each biochemical target are also included in Table 2.  
Figure 1: Schematics of cyanobacterial central metabolism. NADPH and ATP are generated through light reactions. Subsequently, they are used to convert CO\({}_{2}\) into central metabolites, which can be used for biochemical synthesis.