The in situ stabilization of old deposits aims for a lasting and controlled reduction of pollutant emissions from the deposited waste in order to diminish expenditure and the duration of landfill aftercare measures. The stabilization operation so far, observed over a period of 1-2 years at three landfills in Germany, shows that leachate contamination is permanently reduced, biodegradation processes are significantly accelerated and main settlements take place within a short period of time.