Reduced cardiac mortality and morbidity have long been observed in association with omega-3 long chain polyunsaturated fatty acids (LC-PUFA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from fish consumption, without clear physiological explanation. This review seeks to identify mechanisms of action based on evidence: of physiological effects, active components and effective intakes. Fish oil pleiotropic effects reveal actions that are either intrinsic: effects on cardiac function dependent upon membrane incorporation; or extrinsic: indirect cardiac effects through vascular disease. Extrinsic actions require EPA + DHA doses >3 g/day. Intrinsic effects derive from usual dietary intakes, /day and include improved myocardial oxygen efficiency, heart rate, nutritional preconditioning against ischaemic injury, arrhythmias and heart failure. Myocardial Na+ and K+ currents are non-selectively modulated by omega-3 and omega-6 PUFA to stabilise cells in vitro, but not by fish oil-induced membrane change. In contrast, cellular Ca2+ overload involved in ischaemic injury, arrhythmia and spontaneous pacemaker activity are modulated by both dietary fish oil and in vitro omega-3 LC-PUFA. A potential linking role of bioactive epoxy and hydroxy PUFA derivatives requires investigation. Omega-3 DHA predominates over EPA in population intake, is preferentially incorporated into myocardium and is selectively active in heart rate and arrhythmia modulation, but EPA predominates in clinical trials. Myocardial selectivity for DHA and independent intrinsic and extrinsic physiological mechanisms underpinning diverse clinical endpoints can explain some contradictory outcomes of clinical trials. Intrinsic modulation of intracellular Ca2+ handling provides a unifying physiologically plausible basis for intrinsic fish oil actions and insight to nutritional optimisation of cardiac function.