Expansion of extracellular fluid volume is central to the 
pathophysiology of heart failure. Increased extracellular fluid leads to
 elevated intracardiac filling pressures, resulting in a constellation 
of signs and symptoms of heart failure referred to as congestion. Loop 
diuretics are one of the cornerstones of treatments for heart failure, 
but in contrast to other therapies, robust clinical trial evidence to 
guide the use of diuretics is sparse. A nuanced understanding of renal 
physiology and diuretic pharmacokinetics is essential for skillful use 
of diuretics in the management of heart failure in both the inpatient 
and outpatient settings. Diuretic resistance, defined as an inadequate 
quantity of natriuresis despite an adequate diuretic regimen, is a major
 clinical challenge that generally portends a poor prognosis. In this 
review, the authors discuss the fundamental mechanisms and physiological
 principles that underlie the use of diuretic therapy and the available 
data on the optimal use of diuretics.