This disclosure describes genetically modified photosynthetic microorganisms, including Cyanobacteria, that contain one or more mutations or deletions in a glycogen biosynthesis or storage pathway, which accumulate a reduced amount of glycogen as compared to a wild type Cyanobacterium, and which are capable of producing an increased amount of lipids and / or fatty acids. In certain embodiments, the genetically modified photosynthetic microorganisms also contain one or more exogenous genes encoding a diacyglycerol acyltransferase, a phosphatidate phosphatase, and / or an acetyl-CoA carboxylase, and which are capable of producing increased amounts of lipids or fatty acids and / or synthesizing triglycerides.