The article reviews historical basis of Nocturnal Penile Tumescence (NPT) test and the evolution of techniques to perform it. From complex and time-consuming instruments, modern technology has brought us to the use of the Rigiscan system. This is a transportable (home or office usable), computerized method to detect change of penile rigidity and circumference during sleep-related spontaneous-or daytime induced-erections. While the instrument was mainly developed to perform NPT tests, it is nowadays used to record penile rigidity in realtime after the intracavernous injection of vasoactive drugs. In our experience, normal parameters to refer for a NPT evaluation are: number of erectile episodes: 4-5 per night; mean duration od episodes: > 30 minutes; increase of penile circumference: > 3 cm (base loop) and > 2 cm (tip loop); maximal rigidity: > 70% (both base and tip). Reference parameters for realtime monitoring after intracavernous injection of PGEI are: latency of response: < 12 minutes; duration (with a stable plateau): > 30 minutes; constant plateau: > 70%; increase of penile circumference: > 3 cm (base) and > 2 cm (tip). The use of these reference parameters, together with the data coming from other procedures (e.g. penile arterial flow with the Doppler), allows a better diagnostic accuracy in the approach to sexual impotence.