Recovery and testing of representative, undisturbed or intact samples of Municipal Solid Waste (MSW) is difficult, if possible. Therefore, in-situ testing is the most reliable method for evaluation of the material properties of MSW. However, in-situ testing techniques are also subject to limitations that may significantly affect their applicability and consequently reliability of evaluated MSW parameters. Furthermore, because these measurements can only be made after the waste is in place, they are only directly applicable to analysis of existing landfills while extrapolations are required for design of new landfills. Material properties of interest to engineering community include unit weight and moisture content, shear strength, stiffness and compressibility, stress-strain behavior, hydraulic conductivity (saturated and unsaturated), and field capacity. These properties can roughly be categorized into mechanical and physical properties. Similarly, in-situ measurements may be divided into direct measurements of the behavior of MSW and indirect measurements that rely on correlations and/or back analyses to calculate properties of interest. It is generally difficult to categorize a certain in-situ test method as direct or indirect because each test method can directly measure certain properties and indirectly measures other properties. For example, wave propagation surveys are a direct measurement of wave velocity but an indirect measurement of stiffness; similarly, soundings with the Standard Penetration Test (SPT), Cone Penetration Test (CPT) and dilatometer (DMT) can be used to directly identify waste stratigraphy and indirectly to assess shear strength of MSW. Field measurements of MSW properties may also be divided into non-intrusive measurements that do not penetrate the waste mass and intrusive measurements that penetrate the waste mass. Classification of in-situ testing techniques into intrusive and non-intrusive is more distinct and will be used for categorizing in-situ test methods in the remainder of this Chapter. Over the past years, significant improvements in field measurements became possible. These include rapid and inexpensive measurement of ambient vibrations (e.g., REMI), accurate measurement of small deformations with laser beams (e.g., LiDAR), and include abundance of relatively accurate digital terrain measurements generated by regulatory-mandated annual flyovers over landfills. This is in addition to more in-situ test methods being adopted from geotechnical engineering for testing and sounding at landfills, such as SPT and CPT sounding, and testing with devices such as dilatometer and (self boring) pressuremeter. This Chapter aims to provide a critical review of the state-of-the practice of these methods as applied to waste mechanics.