In this provocative review, Commichau and colleagues propose a new unifying theory for the function of the bacterial second messenger cyclic di-AMP (c-di-AMP) in the Gram positive firmicutes. It has been appreciated that the intracellular levels of c-di-AMP impact cell envelope integrity, and indeed c-di-AMP is the only known essential cyclic di-nucleotide. This was thought to occur through modulation of the cell wall. In this article, the authors review what is known about the targets of c-di-AMP and the phenotypes that it controls, leading them to propose that c-di-AMP impacts the envelope by modulating turgor via binding and controlling the activity of osmolyte transporters. This is an interesting model that should spark further research into this important bacterial second messenger.