Advanced anaerobic digestion processes are being developed and used at more wastewater treatment facilities in recent years, especially in North America and Europe. These processes are largely developed to achieve greater performance (i.e., greater volatile solids reduction and increased digester gas production). This paper reviews the performance data and information available for several of these advanced digestion processes, especially the ones most commonly being implemented, evaluated, or proposed in North America. Comparisons are now possible for many of these processes showing the degree of improvement gained by the advanced digestion process over the previous mesophilic digestion process. The paper also describes volatile solids reduction calculations and the methods currently used for these calculations. Related issues are also presented and discussed including product odor, dewatering information, and increased ammonia recycle.