The classification of heart failure by left ventricular ejection fraction (LVEF) has evolved significantly over the past two decades. Originally divided into HFrEF (reduced EF, <40%) and HFpEF (preserved EF, ≥50%), the ESC 2016 guidelines introduced a third category — HFmrEF (mildly reduced EF, 40-49%) — recognising the distinct phenotype and treatment considerations of this intermediate group. The ESC 2021 guidelines further refined this, with LVEF classification now driving four distinct evidence-based treatment pathways. This classification system is fundamental because GDMT benefit is LVEF-dependent — sacubitril-valsartan, CRT, and ICD therapy have only been shown to reduce mortality in HFrEF (LVEF <40%).