Significant advances have been made in our understanding of the mechanical properties of Municipal Solid Waste (MSW) in the past 15 years. MSW property values used in engineering analyses have evolved from unsubstantiated guesses to informed estimates based upon laboratory and field data. Recent findings of importance with respect to MSW properties have included the development of characteristic unit weight profiles, measurement of the at rest lateral earth pressure coefficient (K0) in the field and laboratory, reconciliation of shear strength values from different types of laboratory tests and from back analyses of landfill failures, insight into the role of liquids and pore pressures on shear strength, and evaluation of the dynamic modulus and damping in isotropically consolidated triaxial tests. Despite this progress, many important questions on MSW properties remain unanswered, including the effects of waste composition, waste processing, and waste degradation on most of the mechanical properties of interest, the values of the active and passive earth pressure coefficients for MSW, the cyclic behavior of saturated MSW, and quantitative evaluation of the secondary compressibility of MSW. Furthermore, changes in waste streams, waste processing, and landfill operations suggest that typical values for waste properties will continue to evolve over time. Copyright ASCE 2006.