Environmental Sensor Networks (ESNs) facilitate the study of fundamental processes and the development of hazard response
systems. They have evolved from passive logging systems that require manual downloading, into ‘intelligent’ sensor networks that
comprise a network of automatic sensor nodes and communications systems which actively communicate their data to a Sensor
Network Server (SNS) where these data can be integrated with other environmental datasets. The sensor nodes can be fixed or
mobile and range in scale appropriate to the environment being sensed. ESNs range in scale and function and we have reviewed
over 50 representative examples. Large Scale Single Function Networks tend to use large single purpose nodes to cover a wide
geographical area. Localised Multifunction Sensor Networks typically monitor a small area in more detail, often with wireless adhoc
systems. Biosensor Networks use emerging biotechnologies to monitor environmental processes as well as developing proxies
for immediate use. In the future, sensor networks will integrate these three elements (Heterogeneous Sensor Networks). The
communications system and data storage and integration (cyberinfrastructure) aspects of ESNs are discussed, along with current
challenges which need to be addressed. We argue that Environmental Sensor Networks will become a standard research tool for
future Earth System and Environmental Science. Not only do they provide a ‘virtual’ connection with the environment, they allow
new field and conceptual approaches to the study of environmental processes to be developed. We suggest that although
technological advances have facilitated these changes, it is vital that Earth Systems and Environmental Scientists utilise them.