Tellurium is of increasing technological importance and is a critical element for the development of green energy technologies. Tellurium, with its interplay of four oxidation states, has an exciting – albeit difficult – chemistry which led its discoverer, von Reichenstein, to qualify it as a ‘problematic mineral’. (…) It is now timely to review the situation by discussing all these subjects and bringing together the best interdisciplinary research on tellurium. Find what this Research Front is about here.