A firm erection is the result of a whole series of psychological and physical events. If a problem occurs at any step in the process, the erection may be absent, insufficient or short-lived.
There is therefore a range of possible causes of erectile disorder. These can be grouped into two general types: physical (related to the blood vessels, nerves or hormones) and psychogenic (having an origin in the mind). External factors, like surgery, radiotherapy or drugs, and certain special conditions, like ageing, diabetes and chronic renal failure, can also play a role.
Most erection problems are due to a combination of these factors.