In this chapter we discuss the various phases in site investigation. Phase I investigations are preliminary in nature and are designed to furnish a comprehensive overview of available site information. Phase II investigations consist of site characterization and groundwater monitoring of well installation. Direct methods such as boreholes, piezometers, and geotechnical analysis of soil samples are discussed. While, for indirect methods, we discuss aerial photography, ground penetrating radar, and earth conductivity and resistivity geophysical studies. Furthermore, we discuss issues related to hydrogeochemical investigations. Then, we conclude this chapter with two case studies. The first is related to geoenvironmental investigation at a waste dump site in the United Arab Emirates, while the second is related to the assessment of land salinization in arid lands.