Biofilm-based microalgal cultivation systems represent a different cultivation technology from suspended algal culturing. Unlike traditional suspended cultures, biofilm systems can offer higher biomass concentrations, simplified harvesting, and great potential for reduced energy consumption. Despite these advantages, challenges such as biofilm detachment, fouling, and scalability exist. These challenges must be addressed before large-scale biofilm cultivation can take place. Integrating biofilm-based systems into circular economy models presents an opportunity for resource recovery and climate change mitigation. This chapter explores the fundamentals of biofilm formation, including mechanisms of adhesion, biofilm structure, and important factors influencing growth dynamics. It provides a broad analysis of reactor designs, substrate materials, and operational parameters optimized for diverse applications.