In recent years, concerns regarding to wastewater sludge disposal have increased globally. Productionof sludge has increased recently due to the growth of population. Wastewater sludge classified as a hazardoussubstance, it is not easy to dispose because of certain treatment is required. Typically, sludge is treated atsecured landfill which its limited in availability and expensive. On the other hand, wastewater sludge originatedfrom biological treatment contains organic substance that can be converted into alternative energy resources. Atechnology is needed that is able to reduce the volume of sludge and convert sludge into energy source. Wepresent an overview of various technologies that can be used for conversion of sludge into energy resources.Those technologies are anaerobic digestion, pelletization, combustion, pyrolysis and gasification. Progress andchallenges of each technology is presented in detail. A summary of sludge characteristic originated fromdifferent source will be discussed as well. Emissions and residues that determines the environmental impact isalso considered. Referring to some previous research, it known that wastewater sludge, as unwanted product,has the potential to become future energy resource. This potential can only be used properly if the method ofconversion are effective and efficient.