The magnesium ion (Mg2+) is the most abundant intracellular divalent cation and plays an essential role in a myriad of biochemical processes. Proper Mg2+ homeostasis has been correlated with physiological well-being and Mg2+ deficiency is associated with pathological states including cardiac syndromes, immunodeficiency, epilepsy, migraine, muscular dysfunction and bone wasting. Despite its impact on human health, relatively little is known about the molecular mechanisms that regulate Mg2+ transport and storage. I will describe a series of recent crystal structures of the CorA Mg2+ channel from Thermotoga maritima that have provided the first structural insights into the gating and selectivity mechanisms operating in this unique channel superfamily. CorA is revealed to be a funnel-shaped homopentamer containing a large intracellular regulatory domain linked through an extended alpha-helix to a membrane-embedded ion pore. Functional analysis and molecular dynamics simulations confirm that CorA contains an intrinsic and elaborate Mg2+-sensor which couples cellular Mg2+ status to the regulation of its ion pore. The CorA pore appears to be sealed by at least two gates in its closed state, which suggests that cellular Mg2+ transport is well guarded and very tightly regulated.