Erectile difficulties can be divided into two types: primary and secondary. A man who has never had an erection sufficient for intercourse suffers from primary erectile dysfunction. A man who has some history of normal erections during intercourse, but has developed a persistent inability to obtain or maintain erections, has secondary erectile dysfunction. Secondary erectile dysfuction is more common, and treatment is more successful than in cases of primary erectile dysfunction.

A male's ability to get an erection is an automatic process, a reflex that is not under the man's conscious control (i.e. he can't just "will it" or "make it" happen). When a man has difficulty getting or maintaining an erection, the blood that should flow into the penis and engorge it, making it firm and erect, fails to do so, even though the man may feel excited and stimulated.

The causes of erectile difficulty can be physical, psychological, or a combination of both. As we age, our bodies gradually slow down and it is normal for an older male to take longer (and to require more direct penile stimulation) to become erect than a younger male. Men of any age may experience single instances of erectile failure at some time or another. Poor health, stress, anxiety, fatigue, certain medications, or alcohol consumption may all be responsible for an occasional erectile difficulty.

Erectile dysfunction should not be considered a serious problem unless it occurs consistently or for a long period of time, causing significant stress to the individual or to his relationship.