Anaerobic digestion is one of the most widely used processes for sludge stabilization. The paper reviews the developments of thermophilic sludge digestion including the successful demonstrations at Los Angeles and Chicago. Two recent developments, Multiple Digestion Process as advanced by Torpey in New York and dual digestion system as demonstrated in Hagerstown, Maryland and in Lackawanna, New York are reviewed in detail. It is concluded that the thermophilic digestion process, aerobic, anaerobic or in combination with mesophilic digestion process has been shown to be equivalent or better compared with conventional sludge stabilization processes when operated under research conditions.