This practical manual provides basic theoretical knowledge about fermentative processes, biochemical laboratory techniques, and an arsenal of practical tricks, recipes, do’s, and don’ts for the biogas plant manager. It explains why some popular tests and techniques are unreliable, how to optimize the feedstock’s cost and the energy self-consumption of the digester, and how to analyze experimental error propagation and judge whether a marketing claim or a test result from the literature is correct. All examples are taken from the author’s experience as consultant in managing biogas plants in Italy and Spain. It features a glossary of technical jargon and useful reference tables and formulae. By following the procedures described in this manual, anybody can learn in short time how to become a "bacteria farmer."