Research trend and basic knowledge on the biological degradation of PCE (tetrachloroethylene) were reviewed. At first, anaerobic PCE degradation pathway was introduced, and microorganisms related with biological PCE dechlorination under both anaerobic and aerobic conditions were shown. PCE degradation is readily occurred in anaerobic conditions. Anaerobic PCE degradation was carried out by replacing the strongly combined chloride ion with hydrogen ion. The replacement occurs by electrons from electron donors such as H₂, acetate, lactate, and methanol. The best electron donor for PCE degradation is hydrogen. H₂ produced by the fermentation of carbon source can be used by microorganisms involved in acetogenesis, methanogenesis, sulfidogenesis, dehalorespiration, and iron reduction. These organisms can compete one another in natural ecosystem. Due to the competition, H₂ is sustained at low level. At this low level of H₂, the dechlorination of PCE can be maintained, but is inhibited by highly toxic VC accumulation in anaerobic PCE degradation. Since Dehalococcoides ethenogenes strain 195 can convert PCE into ethene which doesn"t have any toxicity and can further be used as a carbon source, it is very useful for field application. Pseudomonas stutzeri OX1 was able to utilize o-xylene and toluene as a carbon source using toluene-o-xylene monooxygenase for the first time in aerobic coditions. However, it has not further been proven. Based on these basic knowledge of biological PCE degradation, various field applications have been carried out and evaluated. Dehalococcoides related with PCE dechlorination were examined in contaminated sites. In a recent study on a wetland having both anaerobic and aerobic conditions, the effects of soil depth and plant species, and seasonal differences were researched as well as the sequential degradation of PCE. Formate was suggested as an alternative organic compound to efficiently neutralize the PCE degradation product, hydrochloric acid. To improve the PCE biodegradation effectively, the following research directions were proposed: (1) development of aerobically PCE degrading microorganism, (2) development of aerobic/anaerobic hybrid system (3) development of a system capable of maintaining consistent PCE degradation rate regardless of seasonal changes.