The eddy-covariance method represents the only direct way to measure the turbulent fluxes of momentum, temperature, trace gases, and particles between the land surface and the atmosphere. It is a direct measurement of the net carbon-dioxide budget and dry deposition. For that purpose, it is widely used in networks of long-term ecosystem observatories around the world and is the centerpiece of intensive field campaigns investigating biosphere–atmosphere exchange processes. The instrumentation typically consists of a 3-D sonic anemometer/thermometer and one or more additional gas analyzers that are able to measure the high-frequency fluctuations of the scalar to be transported. These instruments are mounted on a meteorological mast to sample the turbulent field under the assumption that eddies are carried along with the mean wind. Further prerequisites of the method are horizontal homogeneity, steady-state conditions and well-developed turbulence. For successful application of the method, a series of quality tests and flux corrections is required, which will be presented together with commonly used instrumentation and postprocessing software. Moreover, we will provide a historical overview and provide guidelines for site selection and setup and the necessary maintenance procedures.