This editorial refers to 'Left atrial function measured by cardiac magnetic resonance imaging in patients with heart failure: clinical associations and prognostic value' † , by P. Pellicori et al., on page 733.The understanding of heart failure (HF) haemodynamics has traditionally focused on left ventricular (LV) structure and function, with the left atrium being viewed simply as a passive transport chamber that empties into the left ventricle. Over the last decade, there has been increasing recognition of the importance of left atrial (LA) structure and function in the pathophysiology of HF. LA volume was first to be established as a biomarker integrating the magnitude and duration of LV diastolic function, and a predictor of cardiovascular outcomes in HF. More recently, LA function has emerged as a novel determinant of clinical status and outcomes in HF, and perhaps an even more robust prognostic marker than LA volume ( Table 1). Pellicori et al. have now explored the prognostic relevance of one summary measure of LA function-LA emptying fraction (LAEF)-by magnetic resonance imaging (MRI) in HF.1 In this single-centre study of 759 patients with suspected HF and in sinus rhythm undergoing cardiac MRI, LAEF was lower on average among 664 patients with the confirmed diagnosis of HF. Lower LAEF correlated with larger LA maximal and conduit volumes, smaller LA reservoir volume, lower LV and right ventricular (RV) ejection fraction, greater LV and RV remodelling, and higher circulating N-terminal pro-brain natriuretic peptide (NT-proBNP) levels. Over a median follow-up of 3-4 years, LAEF, but not LA volume, was independently associated with incident HF hospitalization, cardiovascular mortality, all-cause mortality, and atrial fibrillation in multivariable models that included clinical predictors and NT-proBNP level. Importantly, LAEF provided incremental value in predicting these outcomes, beyond clinical variables alone but not in addition to NT-proBNP levels. The left atrium modulates LV filling and cardiovascular performance by three distinct functions: (i) as a reservoir for pulmonary venous return during LV systole; (ii) as a conduit for passive flow of blood from the pulmonary veins into the ventricle during early LV diastole and diastasis, and (iii) as a pump during late LV diastole that augments LV stroke volume (by 15-30% in healthy individuals and more in the presence of abnormal LV relaxation) (Figure 1). LA reservoir function is influenced by LA compliance as well as LV contraction via descent of the LV base during systole, and RV systolic pressure transmitted via the pulmonary circulation. LA conduit function is inversely related to reservoir function and strongly modulated by LV relaxation and compliance. LA pump function reflects LA contractility and is also dependent on both LA preload (venous return) and LA afterload (LV end-diastolic pressure). Of note, the Frank -Starling mechanism applies to LA mechanics (as with LV mechanics), wherein LA ejection volume increases as LA filling volume increases, ...