Sometimes the semen is produced does not contain any sperm. This is given the name 'Azoospermia'. It can be divided into two forms, obstructive and non-obstructive. The nature of these is so different that it is best to consider them separately:

Obstructive Azoospermia

Obstructive azoospermia is when men produce sperm, but have some physical blockage preventing them from combining the sperm with semen. This can be caused by:

CBAVD (Congenital bilateral absence of the vas deferens). This is a genetic defect, in which the vas deferens, part of the chain of vessels bringing sperm from the testes to be ejaculated, does not form properly. When this affects only one of the two ducts, (congenital unilateral absence of the vas deferens), the condition is only a mild problem, since sperm will still pass through the other duct. The bilateral form leads to complete lack of sperm in the semen.
Cystic Fibrosis. This is very closely connected to CBAVD both are caused by a defect in the same gene. However, they are different conditions: cystic fibrosis is a much more serious condition, affecting more than just the reproductive system. Sufferers of cystic fibrosis usually though not always suffer from CBAVD.
Other genetic causes
Injury. It is quite rare for injury to sever the sperm ducts themselves. More often, injury to this region will cause inflammation and the formation of scar tissue, which will block the ducts.
DES Exposure. Children of mothers who took the drug DES during pregnancy appear to have a higher likelihood of obstructive azoospermia. The mechanism is not yet understood, but this would be only one of many serious reproductive side-effects caused by DES.
Vasectomies. Technically a form of injury- an intentional blocking of the sperm ducts
Tuberculosis. If tuberculosis spreads from the lungs to the reproductive system, infection can cause blockage of the sperm ducts

The usual treatment for obstructive azoospermia is not to fix the ducts, but rather to bypass the problem. The sperm can be removed from the testes or the epididymis, a process described in detail later. One exception is vasectomy reversal, which can now be done with a reasonable likelihood of success (well over 50%, if the reversal takes place within a few years of the vasectomy).

The sperm retrieved in this way can then be injected into the man's partner using either IVF or ICSI (intracytoplasmic sperm injection).

Non-obstructive Azoospermia

The other form of azoospermia, 'non-obstructive azoospermia', is a more serious problem. It is normally caused by hormonal imbalances, such as an unusually high level of Follicle-Stimulating Hormone (FSH), and most sufferers also have unusually small testes.

For many years, doctors considered it untreatable, advising patients to adopt or use a sperm donor. Recently, however, so-called Testicular Sperm Extraction has been used to obtain some sperm in some cases of non-obstructive azoospermia.