Your heart beats 100,000 times a day and hopefully never misses. It is an amazing organ, but its failure to work properly remains a top cause of death. Each heartbeat is initiated by an electrical signal that sweeps over the heart to synchronize the contraction of each of its 5 billion muscle cells to propel 7,000 litres of blood per day around your body to nourish your body’s other 35 trillion cells. Calcium is critical in that normal electrical wave that starts the heartbeat and is the direct intracellular switch that activates contraction and pumping, but also allows the heart to relax and refill with blood between beats. Enhanced calcium levels in the myocyte also mediate the increased strength of your heartbeat when you exercise and also enhances the production of energy (ATP) that is required for that extra cardiac work. Calcium is also at the centre of complex signalling systems that maintain stable cardiac function, but when things go wrong it is also at the centre of clinical problems such as heart failure and arrhythmias. The lecture will discuss the mechanisms by which calcium orchestrates cardiac function in health and is also involved in heart failure and life-threatening arrhythmias.