In municipal wastewater treatment plants (WWTPs), algae could be utilised for cleaning the water and, at the same time, produce a biomass that can be used for energy. Through anaerobic digestion, microalgae can contribute to biogas production when co-digested with sewage sludge. In this paper, previous published results on the co-digestion of sewage sludge and microalgae are summarised and reviewed, and any remaining knowledge gaps are identified. The batch tests currently documented in literature mostly concern digestion under mesophilic conditions, and studies investigating thermophilic conditions are less common. The average biochemical methane potential (BMP) for 29 different mixtures co-digested under mesophilic conditions is 317 +/- 101 N cm(3) CH4 gVS(-1) while the result for 12 different mixtures investigated under thermophilic conditions is a BMP of 318 +/- 60 N cm(3) CH4 gVS(-1). An evaluation of the heat required for increasing the temperature from mesophilic to thermophilic conditions shows that increased methane production under thermophilic conditions can be enough to create a positive energy balance. For a full-scale WWTP, using thermophilic digestion on sludge, or a combination of sludge and microalgae could therefore be of interest. This is dependent on the demands on sanitation of the sludge and the possibilities for heat recovery. Most of the mesophilic investigations indicate a synergetic effect for co-digestion, with enhancements of up to almost 70%. However, the results are uncertain since the standard deviations for some of the BMP tests are in the same order of magnitude as the identified enhancement. Neither of the presented publications provide an understanding of the basic mechanisms that led to higher or lower BMP when microalgae were mixed with wastewater sludge. We, therefore, call for care to be taken when assuming any effects related to the specification of substrates. Microalgae and wastewater sludge have several similarities, and the specific results of BMP in the mixtures relate more to the specifics of the respective materials than the materials themselves. Investigations into semi-continuous processes of co-digestion of microalgae and sludge are scarce. The yields for three co-digestion studies show high variation, with an average of 293 +/- 112 N cm(3) gVS(in)(-1). The available results show strong potential for co-digestion of sewage sludge and microalgae. Further investigations are required to identify optimal conditions for biogas production, and analysis of microalgae implementation on wastewater treatment at a system level is also needed to identify the total mass balance of substrate and nutrient recovery.