Heavy metals are known as harmful pollutants in Wastewater having a negative effect on biological wastewater treatment plant including microorganisms. Biological wastewater treatment is one of the most important parts of wastewater treatment and microorganism has importance role in biological wastewater treatment. Microorganisms effect in metabolism of various organic compounds, nitrogen, phosphorus, Sulphur and other elements. Such microbiological parameters as the number, weight and activity of microorganisms can be good indicators of wastewater contamination with heavy metals. In this book, heavy metals and microorganism is introduced completely and then toxic effect of heavy metal on biological wastewater treatment is reviewed.