Bladder outlet obstruction (BOO) is a common cause of lower urinary tract symptoms (LUTS) in men and women. By definition, BOO is determined urodynamically, assessing the pressure-flow relation during voiding. Since the 1960s much work has been done to standardize the urodynamic definitions of obstruction in men and more recently women. Today, urodynamic testing voiding pressure-flow analysis remains the gold standard for the diagnosis of BOO and the etiology of LUTS. The pressure-flow relation is much better defined in men than in women, but recent work suggests that although the definition of obstruction may differ between men and women, the concept of the pressure-flow relation to diagnose obstruction holds true for both genders.