Involuntary sleep-related erections (SREs) occur naturally during REM sleep in sexually potent men and other mammals. The regularity of their pattern and non-volitional nature made SREs useful clinically for differentiating psychogenic and organic erectile dysfunction (ED) in candidates for surgical intervention. Normative data available for different age groups added to the attractiveness of SRE measurement for clinical decision-making. Clinical SRE testing is less commonly applied today with the advent of minimally invasive medical therapies for ED. Nonetheless, as an objective measure of erectile function, SRE recording for research provides a precise technique for examining the mechanisms of erection and is still conducted to resolve legal disputes. SRE alterations provoked hormonally and pharmacologically are discussed. Different SRE patterns are associated with comorbid factors and some of these are illustrated, described, or both. Recording techniques developed for rats have proved extremely valuable for furthering our understanding of brain centers mediating erectile response. Data from lesion and stimulation studies are examined in the present review, moving us a step closer to understanding the underpinnings of erectile function.