The presentation comprises the results of first chapter of the Best Practice Handbook of ENGINE. It gives an overview of the investigation, exploration, and exploitation of unconventional geothermal reservoirs and Enhanced Geothermal Systems (EGS) to different groups of interest such as engineers, politicians, decision makers from industry. In contrast to the conventional high-temperature steam reservoirs in volcanic environments, unconventional and EGS resources and reservoirs are more difficult to localise and to assess. While volcanic resources are clearly indicated by obvious effects at the surface (e.g. geysers, fumaroles, etc.), unconventional and EGS resources leave mainly indirect traces (e.g. increased surface heat flow). Usually, they are water-dominated systems and characterised by a wide range of production temperatures. The lower temperature limit in unconventional reservoirs is defined by the current technical limitations in conversion of heat into electric energy. An EGS is defined by artificial improvement of the hydraulic conductivity of the reservoir.
This chapter covers the initial phase of an unconventional project. It provides an investigation scheme for possible EGS resources and reservoirs and includes a validation of appropriate exploration techniques in different geo-environments. The Site Investigation is subdivided in Site Screening and Site Characterization. The first one applies a scale-dependent workflow and describes a step-by-step procedure, how to locate a reservoir using different, geoscientific techniques. It introduces different tools and approaches to investigate resources on continental scale. While downscaling to regional and local scales, tools and approaches are adopted to the respective scale and the information of interest on the specific scale. The second topic (Site Characterisation) supports the understanding of physical and chemical processes in the reservoir and proposes techniques to investigate the key properties influencing the productivity of the reservoir. This chapter provides an overview of the necessary evaluation and relevant geoscientific tools to characterise the thermal, hydraulic and chemical conditions of an unconventional or EGS reservoir. A validation of the applied techniques is given in the description of the analogue sites. 
In general, geothermal energy can be analysed on different scales and for various purpose. Accounting for these different scales or levels of interest (e.g. planning of European-wide energy mix or local energy provision), in the BPH the different groups of interest are guided by different entry points.

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