Microalgal biomass is a promising source for biogas production. However, biomass pretreatment is generally a crucial step in disintegrating cell wall structure and allowing it to exploit its potential fully. As a result, methane yield from microalgal biomass is influenced by various factors, including the microalgae species, biomass pretreatment, process operational parameters (such as pH, temperature, and nutrient availability), and the presence or absence of inhibitors. Besides, the co-digestion of microalgal biomass with carbon-rich residues is a suitable strategy to improve biogas production yields. The anaerobic digestion (AD) of microalgae and its integration into biorefineries play a crucial role in closing loops within the current circular economy context. Typically, this integration involves the anaerobic digestion of either microalgal biomass or residual exhaust microalgal biomass (such as that obtained after lipid extraction). However, ensuring the proper integration of both materials involves a rigorous assessment of their environmental impact and economic performance.