Marine cyanobacteria Synechococcus and Prochlorococcus dominate global oceanic photosynthesis and play central roles in carbon cycling. Recent findings have revealed that they encode hydrocarbon biosynthetic pathways, yet their ecological contributions and biotechnological potential remain poorly understood, largely due to challenges in genetic manipulation. In this thesis, I investigated hydrocarbon biosynthesis in marine cyanobacteria using ecological, biochemical, and synthetic biology approaches. I established a diverse culture collection comprising 26 Synechococcus and Prochlorococcus strains, including four novel isolates from Hong Kong coastal waters, and quantified hydrocarbon content to reveal substantial strain-level variability. Based on these measurements, global cyanobacterial hydrocarbon production was estimated to be 118–308 million tons annually. Phylogenetic and structural analyses of the key enzymes AAR and ADO identified conserved and variable residues associated with yield differences. Guided by these insights, I engineered Synechococcus sp. WH7803 to overexpress ado and aar, achieving a 50% increase in pentadecane production without growth penalty. To enable further engineering, I developed an optimized electroporation protocol and established a CRISPR/Cpf1 RNP-based genome editing platform for Synechococcus, while attempts in Prochlorococcus MED4 highlighted ongoing barriers. Together, these findings expand the genetic toolkit for marine cyanobacteria, enhance understanding of their role in oceanic hydrocarbon cycling, and support the use of Synechococcus as a promising chassis for sustainable biofuel production.