Recent technological advancements in microelectronics, wireless communications, intelligent systems and networking have led to the conceptualized notion of a "web" of sensors, interconnected and interacting with one another to provide observations and measurements of geophysical phenomena, and routing data (or information) directly to researchers, scientists and other destinations (including forecast models and data archives). This is the concept that we call the "sensor web." A vision for NASA sensor webs for Earth science is to enable "on-demand sensing of a broad array of environmental and ecological phenomena across a wide range of spatial and temporal scales, from a heterogeneous suite of sensors both in situ and in orbit." This paper takes a high level view of sensor webs, describing their characteristics and giving examples of how NASA sees these systems contributing to our knowledge of the world in which we live. Examples of sensor web prototypes and research highlights will be presented from some of the research projects recently funded by the NASA Earth Science Technology Office and currently underway in universities, industries and NASA research centers throughout the country. The paper concludes by imagining how sensor webs will be used in the future and describing the key research technologies that will be needed to fully realize this vision.