Congestion is a major component of the clinical syndrome of heart failure, and diuretic therapy remains the cornerstone of congestion management. Despite being widely used, there is very limited evidence from prospective randomized studies to guide the prescription and titration of diuretics. A thorough understanding of the pharmacology of loop diuretics is crucial to the optimal use of these agents. Although multiple observational studies have suggested that high doses of diuretics may be harmful, all such analyses are confounded by the association of higher diuretic doses with greater severity of illness and comorbidity. Recent data from randomized trials suggest that higher doses of diuretics may be more effective at relieving congestion and that associated changes in renal function are typically transient. Data from other ongoing trials will continue to inform our understanding of the optimal role for loop diuretics in the treatment of heart failure.