<p>Calcium (Ca²⁺) signalling is essential for cardiomyocyte function, regulating excitation–contraction coupling and excitation–transcription coupling, and contributing to mitochondrial energy production (excitation–bioenergetics coupling). In this Review, we explore the role of Ca²⁺ microdomains, which compartmentalize Ca²⁺ signalling to ensure efficient cardiac function. We first describe the organization of these microdomains, followed by their functional importance, pathological alterations in heart failure and potential therapeutic strategies targeting key Ca²⁺-signalling mechanisms. The dyad, a crucial excitation–contraction coupling microdomain, brings L-type Ca²⁺ channels and ryanodine receptor Ca²⁺-release channels (RYR2) into close proximity, facilitating Ca²⁺-induced Ca²⁺ release for cardiomyocyte contraction. In heart failure, dyadic remodelling and altered Ca²⁺ handling contribute to cardiac contractile dysfunction and arrhythmogenesis. Emerging research shows that dyads are dynamic, rapidly adapting to modulators such as β-adrenergic signalling, offering new therapeutic targets. Under stress conditions, dyadic proteins can translocate to the nucleus to regulate gene expression. In addition to excitation–contraction coupling, which operates on a beat-to-beat basis, Ca²⁺ has additional roles in cardiomyocytes. Nuclear Ca²⁺ regulates the expression of genes related to hypertrophy, including those encoding Ca²⁺ channels and transporters. Disruptions in these microdomains drive pathological remodelling in heart failure and arrhythmias. Understanding Ca²⁺ microdomains is crucial for developing targeted interventions to restore cardiac function while minimizing pro-arrhythmic risks.</p>