present case studies to highlight the variety of archaeological issues that can be solved by applying geophysical methods. T he application of geophysics to archaeology is well known, and is illustrated mainly by its use on major archaeological sites. For a large proportion of these applications, geophysical tools have an acceptable efficiency (Clark, 1990). Preventive archaeology is a specific field of archaeology devoted to the study of remains, which are threatened by development projects. Over the last 20 years, French preventive archaeology has revealed many sites which are rarely well-preserved, and plays a key role in the understanding of our history. The potential destruction of these sites requires the use of the most efficient detection tools and excavation techniques. Far from monumental walls, archaeological sites are usually characterized by ditches, pits, or post-holes, which are often small and sometimes strewn and buried at different depths. The physical characteristics of the remains do not allow them to be completely detected using geo-physical techniques. Since geophysical tools have undeniable limitations, the evaluation of the archaeological potential of a site requires trial trenching. This method is implemented with a mechanic shovel, and involves the excavation of approximately 10% of the field area (Figure 1). At the present time this is the only procedure which allows archaeological features to be interpreted and dated. The decision to excavate only some parts, or the totality of a site, depends on the results of this initial archaeological evaluation.