It is estimated that approximately 50% of the heart failure population has a normal left ventricular ejection fraction, a complex broadly referred to as heart failure with normal left ventricular ejection fraction (HFNEF). While these patients have been considered in epidemiologic studies and clinical trials to represent a single pool of patients, limited more detailed studies indicate that HFNEF patients are a very heterogeneous group, with a number of key pathophysiologic mechanisms. This review summarizes and critically analyzes available data on the pathophysiology of HFNEF, placing it into context with a recently developed diagnostic algorithm. We evaluate the utility of commonly applied echocardiographic measures and biomarkers and integrate mechanistic observations into potential future therapeutic directions.