Pressure-flow studies of voiding contain information about urethral resistance or obstruction and about the detrusor contraction or contractility. A fundamental problem, discussed in this paper, is how to disentangle these two. A second, more difficult, problem is how to evaluate them in a simple, clinically useful way. Plots of detrusor pressure against flow rate during the course of voiding offer a helpful adjunct to conventional urodynamic curves showing changes with time. Many ways of assessing urethral obstruction from these plots have been proposed that have somewhat differing aims but, in practice, produce rather similar results. Agreement on a standard method is needed to allow the comparison of results obtained in different centers. Detrusor contractility has been less extensively studied but methods for assessing it are available.