$Ca^{2+}$ is a universal signalling molecule that affects a variety of cellular processes including cardiac development. The majority of intracellular $Ca^{2+}$ is stored in the endoplasmic and sarcoplasmic reticulum of muscle and non-muscle cells. Calreticulin is a well studied $Ca^{2+}$-buffering protein in the endoplasmic reticulum, and calreticulin deficiency is embryonic lethal due to impaired cardiac development. Despite calsequestrin being the most abundant $Ca^{2+}$-buffering protein in the sarcoplasmic reticulum, viability is maintained in embryos without calsequestrin and normal $Ca^{2+}$ release and contractile function is observed. The $Ca^{2+}$ homeostasis regulated by the endoplasmic and sarcoplasmic reticulum is critical for the development and proper function of the heart.