Characterising a molecule has normally meant isolating it in pure form and determining its constitution (the identity and proportion of the elements it contains) or perhaps its molecular structure (the order and manner in which the elements are connected to each other).

The new science of combinatorial chemistry strives to create molecular libraries containing hundreds or even thousands of new species, which are then subjected to mass screening programs to test for possible biological activity. The individual molecules in such libraries are rarely characterised according to the definition given above. They are in effect "just-in-time" molecules, that could be characterised if the need arose. It is quite possible therefore that very shortly, perhaps 20 million molecules might have been made by science, but not all will have been "characterised" in the sense that their properties are pure molecules are known.