Book excerpt: The need to understand the biological processes that are important for essential aquatic and terrestrial ecosystem function has prompted much research into the field of ecological enzymology. This book presents the two broad areas of application in a compilation of reviews by 21 international experts in their respective fields. The first explores enzymatic activities to assess the processes or mechanisms that operate in a given system, such as the rhizosphere, plant leaves and shoots, soil surfaces, and biofilms. The second considers enzymes or microbial cells as sensors to detect microbial activity and stresses due to pollution, management, or climatic change in both aquatic and terrestrial ecosystems.