[Ca2+]i is used as a signal in many tissues. In this review we discuss the mechanisms that regulate [Ca2+]i and, importantly, what determines their stability. Brief mention is made of the effects of feedback gain and delays on stability. The control of cytoplasmic Ca concentration is shown to be generally stable as Ca pumping is essentially an instantaneous function of [Ca2+]i. In contrast, regulation of the Ca content of intracellular stores may be less stable. One example of this is instability in the control of sarcoplasmic reticulum (SR) Ca content in cardiac muscle. An increase of SR Ca content increases the systolic Ca transient amplitude. This in turn decreases Ca influx into the cell and increases efflux, thereby restoring SR Ca to control levels. This feedback system has an inherent delay and is potentially unstable if the gain is increased beyond a certain level. This instability produces Ca transients of alternating amplitude and may contribute to the clinical syndrome of pulsus alternans.