Erectile dysfunction (ED) in men under the age of 40 was once thought to be entirely psychogenic. Over the last few decades, advances in our understanding of erectile physiology and improvements in diagnostic testing have restructured our understanding of ED and its etiologies. Although psychogenic ED is more prevalent in the younger population, at least 15%-20% of these men have an organic etiology. Organic ED has been shown to be a predictor of increased future morbidity and mortality. As such, a thorough work-up should be employed for any man with complaints of sexual dysfunction. Oftentimes a treatment plan can be formulated after a focused history, physical exam and basic lab-work are conducted. However, in certain complex cases, more testing can be employed. The major organic etiologies can be subdivided into vascular, neurologic, and endocrine. Specific testing should be directed by clinical clues noted during the preliminary evaluation. These tests vary in degree of invasiveness, precision, and at times may not affect treatment. Results should be integrated into the overall clinical picture to assist in diagnosis and help guide therapy.