Reactive nitrogen compounds form a major component of atmospheric acidifying deposition. Species of importance include NO, NO2, HNO3, HNO2, NH3, NO3 & NH4+. A primary task in quantifying the importance of these species is to define the rates and processes controlling their exchange between the atmosphere and the ground. This information is useful both for providing input parameters for models describing the long-range transport and deposition of pollutants, and for estimating long-term budgets for ecosystems and regions. The aim of this study was to examine the exchange processes of these species over a range of European ecosystems. Because of the wide range of chemical species involved, and the range of measurement techniques required, a collaborative project was proposed to pool expertise and facilities of several European research groups. The core of this collaboration was three joint field experiments (Halvergate, pasture; Manndorf, wheat field; Leende, heathland), which were made during the course of the project. In addition, these experiments provided an opportunity to compare measurement techniques being applied in different countries by the groups involved. Analysis of the results from the joint experiments, as well as the individual measurements of groups, has allowed a detailed picture of the exchange of reactive nitrogen compounds to be developed. The results are described extensively in the annex attached to this report, and major findings summarized in the accompanying figures.