Heart Failure is a syndrome describing a pathophysiological state with diverse etiologies. Providing an adequate mechanistic definition is difficult. The current guidelines from the European Society of Cardiology define the diagnosis of heart failure based on three criteria. Patients should have symptoms compatible with heart failure at rest or on exercise. There should be objective evidence of cardiac dysfunction at rest. In doubtful cases, there should be a favourable response following therapy for heart failure. The term diagnosis derives from the Greek words "dia" and "nosi" meaning "through knowledge". It implies that a conclusion is drawn describing the patient's current status based on the available information. This information is commonly based on the symptoms, history, findings at physical examination, results from laboratory tests, and the results from various non-invasive and invasive special examinations. Diagnostic precision is crucial in deciding treatment strategy and this task presents a continuous academic and clinical challenge. Ultimately, the clinical diagnosis of heart failure is based on all the information available to the physicians. No single investigation is specific for this clinical syndrome and management strategies attempt to modify the underlying mechanisms in order to alleviate symptoms and improve survival.