Currently pressure flow urodynamic studies are the only accepted method to establish the diagnosis of detrusor underactivity (du). Symptoms and signs cannot accurately establish the presence of du given the overlap with bladder outlet obstruction (boo). A symptom complex of underactive bladder (uab) may capture patients more likely to have du but the definition of such a symptom complex requires refinement and clinical validation. There are only 2 promising non invasive tests to establish the diagnosis of du, penile cuff urodynamics and ultrasonic measurement of detrusor wall thickness, however both of these approaches are not established and require further study. It is likely that in future the non invasive diagnosis of du will require an approach using a combination of symptoms, signs and a diagnostic test.