Heart failure has been defined in many ways and definitions change over time. The multiplicity of definitions reflect the paucity of our understanding of the primary underlying physiology of heart failure and the many diseases for which heart failure is the common end-point. Fundamentally, heart failure represents a failure of the heart to meet the body’s requirement for blood supply for whatever reason. It is thus a clinical syndrome with characteristic features – not a single disease in its own right. The syndrome includes symptoms and signs of organ underperfusion, fluid retention and neuroendocrine activation. The syndrome arises from a range of possible causes of which ischaemic heart disease is the commonest. From the point of view of a clinician, the underlying pathology will determine treatment options and prognosis. The extensive range of possible aetiologies present a diagnostic challenge both to correctly identify the syndrome amongst all other causes of dyspnoea and to identify the aetiology, allowing optimization of treatment.