Activated sludge (AS) process is the most widely process in wastewater treatment because of its simple operation and good treatment effect. Sludge bulking and sludge foaming, known as the “cancer” of wastewater treatment plants (WWTPs), affect the solid-liquid separation process of the secondary sedimentation tank and the stability of the microbial biomass in the biological reaction tank, which have long troubled the operation of WWTPs. The definition, classification, characterization methods, filamentous bacteria, and control methods of sludge bulking and sludge foaming were compared and analyzed as comprehensively as possible in this review. In addition, the future research direction and control strategy of sludge bulking and sludge foaming were discussed. It is expected to provide helpful information for future research on sludge bulking and sludge foaming.