The effectiveness of in situ aeration of landfills depends heavily on the permeability of the waste body and the heterogeneity of the flow paths. In the Trace & Test project, we are working with Delft University of Technology and the Dutch Foundation for Sustainable Landfill Management (https://duurzaamstorten.nl/en/) to analyse the flow paths for gas in waste on a full-scale basis. This involves pressure field tests and gas and liquid tracer tests.