Clays play an important role in oil and gas industry. To begin with we have to deal with them in a geological context, investigate their potential from hydrocarbon generation, possible migration paths and reservoir points of view. They exert tremendous effect on the technological success and economy of drilling exploration of hydrocarbons through well bore stability and drilling safety issues. The former refers to avoiding caverns in the borehole while the latter means the recognition of overpressurized layers before hitting them. A fundamental use of clays, especially montmorillonite, has a long history and is the drilling mud itself. Bentonite gained a central role in mud technology due to its rheological characteristics as a 3D structure building agent. Upon reaching the layers of interest we can have samples and they can be analyzed from geological, geochemical and petrophysical viewpoints. The subject of investigation might be the types of clays and their qualitative and quantitative description, the source rock and migration path identification and geochemical and petrophysical correlations. When we have a producing formation, clays are still in the centre of our attention because they might cause permeability impairment through fines migration and swelling caused by incompatible fluids used in well treatments or other operations. Therefore their analyses - there are a number of ways to analyze them by means of e.g. XRD, SEM, FT-IR or other special tests - yield very important information for planning and executing well or field scale operations, thus avoiding or minimizing clay induced formation damage and other drilling and production problems.