Today, the majority of the world's energy is provided by fossil fuels. Natural energy resources soon will be consumed as a result of rising energy needs of the growing population, leading to a global energy crisis. The insecurity in the energy market also affects the global economy negatively and forces the governments to investigate renewable energy alternatives such as bioenergy. Bioenergy technologies can reduce greenhouse gas (GHG) emissions significantly and can serve to satisfy many forms of energy demand. Microbial fuel cells (MFCs) are one of the bioenergy technologies converting the chemical energy in the bonds of organic wastes into electricity through the biocatalytic reactions of microorganisms. Since the microorganisms present in MFCs utilize organic matter while producing electricity, the use of wastewater and sewage sludge as substrate (fuel) makes MFCs not only a renewable energy technology but also a treatment alternative. Especially, considering the energy consumption of conventional wastewater and sludge treatment systems, MFCs offer a sustainable solution that supply the energy required while achieving high levels of treatment. In addition to these, MFCs can be used in sensors, biohydrogen production and bioremediation. However, the technologic and economic problems with MFCs limit their large scale applications. The purpose of this study is to analyze the recent studies on MFCs and evaluate the outstanding operational parameters and investigate the energy production and efficiency in MFCs fed with different types of wastewater sludge.