Detrusor underactivity (DU) and underactive bladder (UAB) are the two faces of the same coin, the first based on urodinamic data and the second on lower urinary tract symptoms (LUTS). While a worldwide-accepted urodynamic definition is lacking, particularly among women, symptoms are not specific. Hence, a high degree of suspicion is necessary during consultations, in particular when investigating LUTS in elderly or diabetic patients and in men with possible BOO. Mechanism affected by these conditions include sensory pathways, parasympathetic innervation and the urethral-bladder reflex. No treatment exists besides intermittent catheterization and prostatectomy in men if BPE-BOO is present.