Although Fura-2 has been widely used as the preferred excitation ratioable calcium indicator, it has certain limitations, e.g., lower sensitivity compared to the single wavelength calcium dyes such as Fluo-8^® and Cal-520™. AAT Bioquest has recently developed Fura-8™ to improve the calcium response of Fura-2. As demonstrated in Figures 1.1 and 1.2, Fura-8™ AM is more sensitive to calcium than Fura-2 AM. In addition, Fura-8™ has its emission shifted to longer wavelength (Em = 525 nm). Fura-8™ might be also used for the flow cytometric analysis of calcium in cells due to its excellent excitation at 405 nm that perfectly matches the violet laser line of flow cytometer.