Anaerobic digestion (AD) is a biological process in which various types of biomasses are converted mainly into gaseous product called biogas. The importance of biogas lies in its composition: methane is the major compound, being used as a biofuel. Although the history of biogas dates back to more than two thousand years ago, the first biogas plant was built in the middle of the 19th century. AD started to be applied for wastewater treatment and biogas production in the early 1900s, and soon after, some researchers realised that biogas could be produced from the microalgae generated during the secondary treatment of wastewater. Afterwards, an increasing number of studies about the AD of microalgae have been published worldwide. This chapter introduces the basic principles of AD and describes the influence of some of the most important operating parameters during biogas production from microalgae.