Vast networks of meteorological sen-sors ring the globe, providing continuous measurements of an array of atmospheric state variables such as temperature, humid-ity, rainfall, and the concentration of car-bon dioxide [New etal., 1999; Tans etal., 1996]. These measurements provide input to weather and climate models and are key to detecting trends in climate, greenhouse gases, and air pollution. Yet to understand how and why these atmospheric state vari-ables vary in time and space, biogeoscien-tists need to know where, when, and at what rates important gases are flowing between the land and the atmosphere. Tracking trace gas fluxes provides information on plant or microbial metabolism and climate-ecosystem interactions. The existence of trace gas flux networks is a relatively new phenomenon, dating back to research in 1984. The first gas flux mea-surement networks were regional in scope and were designed to track pollutant gases such as sulfur dioxide, ozone, nitric acid, and nitrogen dioxide. Atmospheric obser-vations and model simulations were used to infer the depositional rates of these haz-ardous chemicals [Fowler etal., 2009; Mey-ers etal., 1991]. In the late 1990s, two addi-tional trace gas flux measurement networks emerged. One, the United States Trace Gas Network (TRAGNET), was a short-lived effort that measured trace gas emissions from the soil and plants with chambers distrib-uted throughout the country [Ojima etal., 2000]. The other, FLUXNET, was an interna-tional endeavor that brought many regional networks together to measure the fluxes of carbon dioxide, water vapor, and sen-sible heat exchange with the eddy cova-riance technique [Baldocchi etal., 2001]. FLUXNET, which remains active today, cur-rently includes more than 400 tower sites, dispersed across most of the world's cli-matic zones and biomes, with sites in North and South America, Europe, Asia, Africa, and Australia. More recently, several spe-cialized networks have emerged, including networks dedicated to urban areas (Urban Fluxnet), nitrogen compounds in Europe (NitroEurope), and methane (MethaneNet). Technical Aspects of Flux Networks Eddy covariance flux measurements are the preferred method by which biogeoscien-tists measures trace gas exchange between ecosystems and the atmosphere [Baldoc-chi, 2003]. In the eddy covariance tech-nique, trace gas fluxes are calculated from the instantaneous changes in the vertical wind velocity and atmospheric gas concen-tration. A key attribute of the eddy covari-ance method is its ability to measure fluxes in situ with minimal disturbance to the environment, at a spatial scale of hundreds of meters, and on time scales spanning hours, days, and years. For the eddy covariance method to work, trace gas sensors must be able to respond to fluctuations in atmospheric gas concen-trations over as little as a tenth of a second, maintain a stable calibration, possess a high signal-to-noise ratio, and, in cases where pumps are needed to move air to the sen-sor, have access to a power line. The cur-rent generation of carbon dioxide and water vapor sensors easily meets these criteria, and a revolution in instrument development is producing trace gas sensors capable of measuring a broad suite of compounds at high sampling rates with high sensitivity and precision. Those measuring stable isotopes of carbon, oxygen, and carbonyl sulfide can help partition fluxes between the vegetation and the soil. Those measuring methane and nitrous oxide can assess microbial activity in the soil. And measurements of hydrocar-bons, ozone, and nitrogen oxides can assess