Book excerpt: This book provides a comprehensive understanding of a highly innovative method of natural wastewater treatment using advanced in-groundbioreactors called Eco-Engineered Bioreactors (EEBs), and traces their evolution from the earliest aerated gravel bed versions once known as Engineered Wetlands (EWs) and now known as BREW Bioreactors (BBRs) all the way to today’s wide slate of aerobic and anaerobic varieties. Treatment using EEBs involves passing wastewaters through excavated basins in which they contact fixed films of microbial consortia on permeable substrate media. Written from the perspective of ecological engineers designing EEBs, this guide covers updated information on the state-of-the-art for EEBs, covering their morphologies, testing methods, designs, operations, and microbiology.