The inexorable shift toward renewable energy sources has catapulted third-generation (3G) biorefineries, which utilize algal biomass, into the spotlight as the greenest and most sustainable energy producers. Algal biomass is a top-performing feedstock because of its fast growth rate, high photosynthetic efficiency, and nonfood crop competition. This chapter comprehensively explores the current progress in algal feedstock cultivation and downstream processing specifically designed for 3G biorefineries. It first briefly introduces algal taxonomy, pointing out the potential of microalgae as well as macroalgae. The efforts made to find new approaches in cultivation systems, such as new photobioreactor designs, wastewater-based systems, and the employment of industrial flue gases in order to sequester CO₂, have been examined. Besides, the chapter delves into possible methods of improving the nourishment conditions to the utmost by optimizing light intensity, temperature, materials, and biostimulants. Some of the most recent achievements in energy-saving harvesting and dewatering technologies—such as filtration, flotation, and membrane-based separations—are included. Several different routes for the processing of algal biomass into renewable transportation fuels, including biodiesel and bioethanol, as well as biogas, have been developed; moreover, the high-value coproduct extracts such as pigments and bioplastics point out the integrated and holistic nature of 3G biorefineries. Such an in-depth review and coverage of these topics vividly illustrate the promising future of algae-based systems in cultivating the bioeconomy and substantially contributing to circular and sustainable industrial practices.