A biogas plant is a container of a slurry of micro-organisms and wet organic matter that produces methane and carbon dioxide. The methane-producing microbes are anaerobic, i.e. they do not work when oxygen is present. A cow’s gut contains a population of microbes that include anaerobic methanogens, so cattle dung is often used as feedstock or as a starter for the digestion of other types of organic material. Since these microbes are adapted to a cow’s gut, they run best at 35°C (mesophylic), although some also work at 55°C (thermophyllic). In colder climates the container must be insulated and/or heated to maintain this temperature. The container must be gas tight to retain the gas as it is given off and also to exclude air. The process of digestion is complex, as it relies on many different microbes working symbiotically. The micro- organisms include bacteria archaea, fungi and yeasts. The process can be divided into three or four stages: 1. hydrolysis (carbohydrates, fats and proteins are converted into soluble compounds) 2. acidogenesis (the soluble compounds are converted into fatty acids) 3. acetogenesis (fatty acids are converted into volatile fatty acids [VFAs], such as acetic acid) 4. methanogenesis (VFAs are converted into methane [CH 4 ] and carbon dioxide [CO 2 ]) VFAs have bad odours and cause rotting food to smell obnoxious. When VFAs are converted into biogas, the bad odours are removed. The matter that cannot be digested can be converted into compost, which should then be almost odour-free. Biogas technology has a long history, especially in countries such as China, India and Nepal. In UK, the rest of Europe and USA, it had a brief burst of interest in the early 1980s, but was seen as uneconomic, when the oil price dropped. Since the 1990s, there has been a strong revival of interest in Germany, followed by Sweden, Denmark and Austria. The UK was slow in following their lead. However they have followed other European countries by providing subsidised feed-in tariffs (FiTs) for operators who generate electricity and grid gas from biogas. These FiTs are now disappearing, either as part of a planned reduction, as in many countries in Europe, or as a sudden change in policy, as in the UK. Biogas is a fuel gas that can be used in gas burners, for cooking and heating, or in internal (and external) combustion engines to give shaft power or to drive an electric generator. Traditionally the main feed material for biogas plants is animal dung, especially cattle dung. Animals process biomass materials, to release energy for their use, while cattle dung contains the right microbes. Raw biomass materials will generate more biogas per kg than animal dung, but needs to be pre-processed. Pre- processing can improve the process of digestion and allow a much wider range of materials to be digested. Further research is being done in this area, especially using a Counter Flow Leach Bed (CFLB) predigester. The two books, “Running a Biogas Programme” (1988) and “Small-scale Rural Biogas Programmes” (2015) provide a detailed description of the technology and how it can be used to help people in many countries, especially LDCs (Less Developed Countries). The books include detailed biogas plant designs and how they are used in biogas extension programmes in many countries.