In forested areas precipitation chemistry undergoes various chemical changes as the precipitation passes through the forest canopy. These changes depend on a number of factors such as leaching from needles or leaves, dry atmospheric deposition prior to the onset of precipitation, initial chemistry of precipitation reaching the forest canopy, meteorological as well as air quality characteristics. This paper summarizes the results of a study started in 1985 at three forest sites intensively instrumented in the Swiss lowlands, pre-Alps, and Alps, where several hundreds of samples of precipitation above and below the forest canopies were analyzed and compared with the simultaneous information on meteorological and air quality parameters.