At the Waldstein-Weidenbrunnen site, several techniques for data quality control were developed and tested and later on applied at European FLUXNET sites. The history of this development and the specific results for the site form the subject of this chapter. These data quality criteria include integral turbulence characteristics, which are dependent on heterogeneities in the footprint area and inside the canopy. Furthermore, footprint models were applied to determine the footprint climatology and to link these models with the data quality of eddy covariance data. This tool was also applied to find the optimal period for the application of the planar-fit rotation method. The energy balance closure was found to be about 80 % in all periods. These findings were summarized as a schema for data quality control and characterization of FLUXNET sites.