Control of protein phosphorylation-dephosphorylation events occurs through regulation of protein kinases and phosphatases. Phosphatase type 1 (PP-1) provides the main activity of serine/threonine protein phosphatases in the heart. Inhibitor-1 (I-1) was the first endogenous molecule found to inhibit PP-1 specifically. Notably, I-1 is activated by cAMP-dependent protein kinase A (PKA), and the subsequent prevention of target dephosphorylation by PP-1 provides distal amplification of β-adrenoceptor (β-AR) signalling. I-1 was found to be down-regulated and hypo-phosphorylated in human and experimental heart failure but hyperactive in human atrial fibrillation, implicating I-1 in the pathogenesis of heart failure and arrhythmias. Consequently, the therapeutic potential of I-1 in heart failure and arrhythmias has recently been addressed by the generation and analysis of several I-1 genetic mouse models. This review summarizes and discusses these data, highlights partially controversial issues on whether I-1 should be therapeutically reinforced or inhibited and suggests future directions to better understand the functional role of I-1 in physiological and pathological β-AR signalling.