Thanks to the coordinated infrastructure composed by a distributed collection of different resources, sensor webs currently represent a very effective solution to earth observation. These global entities are able to coordinate networks of heterogeneous sensing platform, such as spacecrafts, airborne instruments, or ground-based devices. In the framework of technologies operating at local level, wireless sensor networks (WSNs) are characterized by small, low-cost, and autonomous devices that collect data about physical quantities in a distribute and pervasive fashion. This paper is aimed at reviewing some innovative implementations of WSNs for earth observation purposes.