This disclosure describes genetically modified photosynthetic microorganisms, e.g., Cyanobacteria, that contain one or more exogenous genes encoding a phospholipase and/or thioesterase, which are capable of producing an increased amount of lipids and/or fatty acids. This disclosure also describes genetically modified photosynthetic microorganisms that contain one or more exogenous genes encoding a diacyglycerol acyltransferase, a phosphatidate phosphatase, and/or an acetyl-CoA carboxylase, which are capable of producing increased amounts of fatty acids and/or synthesizing triglycerides, as well as photosynthetic microorganism comprising mutations or deletions in a glycogen biosynthesis or storage pathway, which accumulate a reduced amount of glycogen under reduced nitrogen conditions as compared to a wild type photosynthetic microorganism.