The general approach of the fourth generation of biofuels is to use genetically modified photosynthetic microorganisms to produce organic compounds directly from CO2, water, and sunlight (Figure ), which avoids biomass cultivation, harvesting, and processing. Recent advances in the metabolic engineering of cyanobacteria have been reported to produce shortchain olefins, including isoprene. One may also go via even shorter hydrocarbons, e.g., ethylene produced photosynthetically, to form conjugated dienes such as 2,4-hexadiene. Furthermore, metabolically engineered non-photosynthetic microbes can produce other conjugated dienes (e.g., 1,3-butadiene) and trienes, and incorporation of the corresponding genes into photosynthetic microorganisms might be a viable route.