The depletion of fossil fuel reserves driven by rapid urbanization and the escalating impacts of global climate change has intensified the search for renewable and environmentally friendly alternatives. Petrochemical products such as diesel and synthetic plastics are nonrenewable and nonbiodegradable. They contribute significantly to environmental hazards. In this context, photosynthetic microalgae have emerged as a suitable feedstock for biobased products, owing to their global distribution, high biomass productivity, and capacity to sequester atmospheric carbon dioxide. Through photosynthesis, microalgae convert sunlight and carbon dioxide into a range of biomolecules, including carbohydrates, proteins, and lipids. Notably, microalgal lipids can be converted into biodiesel, which is composed of monoalkyl fatty acid esters, while polyhydroxyalkanoates (PHAs) are naturally occurring polymers in microalgae that can be transformed into bioplastics. These microalgal-derived products are sustainable and biodegradable alternatives to the petrochemical-based diesel and plastics, which are major contributors to greenhouse gas emissions, environmental pollution, and long-term ecological degradation. Numerous microalgal species are rich in lipids and PHAs, rendering them highly suitable for large-scale bioproducts production. The effective utilization of microalgal biomass facilitates its integration into circular bioeconomy frameworks and thereby supports the advancement of green energy technologies aimed at reducing emissions and enhancing resource efficiency. This chapter provides a comprehensive review of lipid and PHA biosynthesis in microalgae, detailing their metabolic pathways, molecular composition, and physicochemical characteristics, while emphasizing their potential in the sustainable production of biodiesel and bioplastics.