Missing persons are many and greatly varied; they can become missing for any number of different reasons. Someone who has disappeared for a long time, with no apparent reason, is generally termed as a "missing" person. Typically, a photograph of the missing person is posted on postcards, bulletin boards, and websites, with a contact number to call if anyone comes see an individual resembling the picture. The reasons for such disappearances can be varied: runaway; throw away (abandoned children); kidnapping or abduction; slavery; wage slavery; child pornography; prostitution (often in combination with drug addiction); child stealing; serfdom; sweat shop work service; unfree labor; shanghaiing; or simply running away.

In the United States, there is an average of over 100,000 reported cases of people missing annually; eight of every ten cases are children. Generally, people have a false impression that a person must be missing for 72 hours (three days) before legally termed as a "missing" person. However, this is seldom the case. In circumstances where there is evidence of unusual absence or violence, law enforcement agencies regularly emphasize the importance of starting a prompt investigation. Usually, if a person has gone missing for seven years, without any evidence to prove otherwise, he or she is deemed as "legally dead."

Every year, around the world, a million people are reported missing. About nine of every ten missing people are eventually found. The rest are labeled "missing." In America, the intensity of the law enforcement in the area of missing people has increased in recent times.