In Lower Saxony three full-scale plants for the mechanical-biological pretreatment (MBP) of residual municipal solid waste (RMSW) prior to landfilling are now in operation. The three plants support a wide range of mechanical and encapsulated biological process techniques. In addition to biological degradation, pretreatment allows mechanical separation of iron fractions and refuse derived fuel (RDF). This causes a strong reduction in the need for landfill sites and hence lowers landfill emissions. To describe the efficiency of biological stabilization of RMSW several chemical-physical or biological methods for analysing solid matter or eluate can be used. On the one hand, chemical-physical methods determine a large portion of the total organic carbon (TOC) of pretreated wastes, but often do not allow a differentiation between mobile (with respect to degradable) and immobile (with respect to non-degradable) portions. On the other hand biological parameters explain only a small portion of the TOC, but are well suited devices for qualitative evaluation of landfill reactions and emissions. First results from separate landfill compartments show lower hydraulic permeability, higher landfilling density, lower organic and nitrogen concentrations in leachate of Landfilled MBP wastes compared to untreated wastes. An acid phase with high organic leachate concentrations does not occur. This will create new requirements for landfilling techniques.