Although thiamin was the first vitamin discovered over a century ago, it is only within the last decade that its metabolism has begun to be unraveled. Over the last few years, several structural and biochemical studies have provided insight into the unprecedented mechanisms of the proteins involved, revealing some remarkable biochemistry. Thiamin biosynthesis is particularly unusual in eukaryotes (fungi and plants) in that it cannibalizes essential cellular cofactors and relies on single turnover proteins, which succumb to enzymatic suicide. Here we provide an overview of recent structural studies that have advanced our understanding of this vital metabolite and question whether the single turnover proteins act to monitor the level of the essential elements used as substrates.