Do we need impotence testing? Yes, it is the clinician's obligation to establish the etiology of impotence: end organ vascular failure vs neurologic dysfunction vs psychosexual dysfunction, classify the severity of that dysfunction, and select a therapy that is not only acceptable to the patient but also addresses his pathology. The most commonly utilized diagnostic tests for erectile dysfunction are outlined in this monograph. Nocturnal erections are evaluated by tests commonly known as nocturnal penile tumescence (NPT) studies. NPT has been measured by each of the following methods: stamp test, Snap Gauges, strain gauges, NPTR (Rigiscan, Osbon Medical Systems), and sleep lab NPTR. Normal Nocturnal Penile Tumescence and Rigidity (NPTR) depends on both the integrity of the corticospinal efferents to the penis and vascular responsiveness of the penile tissues to those nerve signals. When nocturnal erections are of appropriate duration and strength the central and peripheral neuroeffectors and intra-corporal regulators of penile hemodynamics are intact. Unfortunately, abnormal NPTR is of little value in determining the etiology or classifying the severity of vascular impotence; the most prevalent kind of end organ failure. The sacral reflex arc of erection consists of somatosensory afferents via the dorsal and pudendal nerves and autonomic efferents via the pelvic and cavernous nerves. These afferents have been measured indirectly by somatosensory evoked potentials (SSEP) and bulbocavernosus reflex latency (BCR). Penile EMG's have recently been recorded, corporal cavernosal smooth muscle electrical activity: CC-EMG. This technology is far from standardized; computer-assisted interpretations of penile electrical potentials may eventually differentiate afferent nerve pathologies so long inferred in: diabetes, spinal cord injury and following radical pelvic surgery. Numerous diagnostic tests have been employed to evaluate penile hemodynamics: penile plethysmography, penile blood pressures, penile brachial index, selective internal pudendal pharmacoangiography, Doppler sonography, dynamic infusion cavernosometry/cavernosography, nuclear washout radiography, and color duplex Doppler ultrasound. Insufficient corporal veno-occlusion is implicated in up to 50% of patients. The diagnosis and demonstration of venous leakage requires complete smooth muscle relaxation. Veno-occlusive dysfunction is associated with poorly sustained erections; this pathology has traditionally been evaluated with Dynamic Infusion Cavernosometry and Cavernosography. DICC is an invasive test, and is now primarily reserved for patients considering the option of vascular reconstructive procedure. Pharmacotesting consists of intracavernous injection and visual rating of the subsequent erection; the test is the most commonly used office procedure for diagnosing erectile dysfunction. It is simple, minimally invasive, and performed without monitoring equipment. Hemodynamic investigations suggest that a positive injection test is associated with normal veno-occlusion, but not necessarily with normal arterial function. When the penile response to pharmacotesting is suboptimal or equivocal, diagnostic testing with duplex Doppler assessment should be performed. The penile blood flow study (PBFS) provides an objective, minimally invasive evaluation of a suboptimal/equivocal erectile response.