Calsequestrin is by far the most abundant Ca²⁺-binding protein in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle. It allows the Ca²⁺ required for contraction to be stored at total concentrations of up to 20mM, while the free Ca²⁺ concentration remains at ∼1mM. This storage capacity confers upon muscle the ability to contract frequently with minimal run-down in tension. Calsequestrin is highly acidic, containing up to 50 Ca²⁺-binding sites, which are formed simply by clustering of two or more acidic residues. The K_d for Ca²⁺ binding is between 1 and 100μM, depending on the isoform, species and the presence of other cations. Calsequestrin monomers have a molecular mass of ∼40kDa and contain ∼400 residues. The monomer contains three domains each with a compact α-helical/β-sheet thioredoxin fold which is stable in the presence of Ca²⁺. The protein polymerises when Ca²⁺ concentrations approach 1mM. The polymer is anchored at one end to ryanodine receptor (RyR) Ca²⁺ release channels either via the intrinsic membrane proteins triadin and junctin or by binding directly to the RyR. It is becoming clear that calsequestrin has several functions in the lumen of the SR in addition to its well-recognised role as a Ca²⁺ buffer. Firstly, it is a luminal regulator of RyR activity. When triadin and junctin are present, calsequestrin maximally inhibits the Ca²⁺ release channel when the free Ca²⁺ concentration in the SR lumen is 1mM. The inhibition is relieved when the Ca²⁺ concentration alters, either because of small changes in the conformation of calsequestrin or its dissociation from the junctional face membrane. These changes in calsequestrin's association with the RyR amplify the direct effects of luminal Ca²⁺ concentration on RyR activity. In addition, calsequestrin activates purified RyRs lacking triadin and junctin. Further roles for calsequestrin are indicated by the kinase activity of the protein, its thioredoxin-like structure and its influence over store operated Ca²⁺ entry. Clearly, calsequestrin plays a major role in calcium homeostasis that extends well beyond its ability to buffer Ca²⁺ ions.