The production of excess sludge by biological wastewater treatment processes has been a serious issue for the operation of wastewater treatment plants (WWTP) on both the economic and environmental sides. To reduce the sludge volume by the separation of water from solid matter, the sludge dewaterability needs to be improved through conditioning processes. Many conditioning methods have been developed and applied for this purpose. Among them, oxidization techniques have many advantages including lower cost, higher efficiency, and lower environmental impact. This paper reviews the recent progress of sludge conditioning techniques and the basic mechanisms involved. Especially, a detailed review and discussion are dedicated to the oxidization techniques and their applications to sludge dewaterability improvement.