Of all naturally occurring clays, bentonites are arguably the most interesting, versatile and useful. This issue of Elements describes how these fascinating materials occur and how they are used in all manner of applications. Composed predominantly of swelling minerals (smectites) and formed mainly from the alteration of volcanoclastic rocks, bentonites are used by geologists for stratigraphic correlation. Bentonite deposits are mined worldwide as they are commercially very valuable. Because of their physicochemical properties, bentonites are used in a wide variety of industrial applications, including the drilling industry, foundries, civil engineering, adsorbents, filtering, etc. Recent formulations of polymer–smectite nanocomposites have been used in industry to make new materials with amazing properties and diverse applications. Bentonites play an important role in the protection of the environment from industrial waste and pollutants and have also been used in medical applications in human health.