The study of illite may seem rather narrow in scope, but we hope to demonstrate that this is not completely true. In fact this assemblage of published and unpublished research tries to analyze the accumulated knowledge and interpretations in order to make a summary of illite occurrence, to define more precisely what illite is and then to establish its place in a geological-environmental context. The information gathered since the 1950s has made proper identification and interpretation much easier as time has passed and more work has been done in fields where little information was previously available. However, research on the identification and diagnostic of illite occurrence and its related clay mineral suites stagnated somewhat in the 1980s. It seemed that further investigation would bring little new information. This was especially true in studies of soil clay mineralogy. The research activity in agronomy departments in institutions over the world has almost come to a halt. This is largely due to the difficulty in further pursuing clay mineral identification with the tools at hand, as well as a shift of interest from soil to plants in agronomic research. The statics of clay mineral identification and the slow mineral change observed in studies of sedimentary series, occurring on the scale of millions of years, seems to have discouraged people from observing the changes possible in active soil sequences. Silicates were assumed to be stable, or at least of such slow kinetic reaction that their existence was a given in soil clay problems. Interest in clay minerals shifted to problems related to sedimentary basins subjected to burial diagenesis and related problems of petroleum generation. Here the general scheme was established and one had only to apply the recipe to a given problem.