Biogas contains not only methane, carbon dioxide and water, but may also contain other compounds in small amounts. These impurities can affect the equipment for biogas utilisation, by causing corrosion and mechanical wear, but may also lead to unwanted emissions. Depending on how the biogas is going to be utilised it often needs some form of cleaning to reduce the amount of unwanted impurities. The effects that different impurities may cause when biogas is utilised and technologies to reduce their concentrations are reviewed in this chapter.