The document provides an overview of anaerobic digestion, which is a natural process where microorganisms break down organic materials in an oxygen-free environment, producing biogas. It discusses the history of anaerobic digestion from its discovery in the 1600s to current applications. The document also outlines the multi-step digestion process and different technologies used, including liquid, high solids, plug flow, micro, and high rate digestion as well as co-digestion.