The aim of this chapter is to discuss evidence concerning the many roles of calcium ions, Ca2+, in cell signaling pathways that control heart function. Before considering details of these signaling pathways, the control of contraction in ventricular muscle by Ca2+ transients accompanying cardiac action potentials is first summarized, together with a discussion of how myocytes from the atrial and pacemaker regions of the heart diverge from this basic scheme. Cell signaling pathways regulate the size and timing of the Ca2+ transients in the different heart regions to influence function. The simplest Ca2+ signaling elements involve enzymes that are regulated by cytosolic Ca2+. Particularly important examples to be discussed are those that are stimulated by Ca2+, including Ca2+-calmodulin-dependent kinase (CaMKII), Ca2+ stimulated adenylyl cyclases, Ca2+ stimulated phosphatase and NO synthases. Another major aspect of Ca2+ signaling in the heart concerns actions of the Ca2+ mobilizing agents, inositol trisphosphate (IP3), cADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate, (NAADP). Evidence concerning roles of these Ca2+ mobilizing agents in different regions of the heart is discussed in detail. The focus of the review will be on short term regulation of Ca2+ transients and contractile function, although it is recognized that Ca2+ regulation of gene expression has important long term functional consequences which will also be briefly discussed.