Many people who decide to become private investigators already have experience in a related field. They may have served in a branch of the military or worked as police officers. Others have experience in crime-scene investigation or surveillance. While this experience can be helpful, it doesn't entirely replace education and training.

In most cases, a person learns to be a private investigator through apprenticeship with an experienced investigator or formal instruction. Either on the job or in a classroom, the future investigator learns about:

Some investigators also use DVDs and distance-learning programs to continue their educations.

In many parts of the world, education and training are only a first step -- becoming a private investigator also requires applying for and obtaining a license. But the process a person has to go through, or whether licensure even exists, varies from place to place. England and Wales, for example, have no official licensing procedure. However, the Security Industry Authority, which regulates private security in Great Britain, conducted research in 2005 and 2006 which may eventually lead to licensure for private investigators.

In the United States, each state has its own licensing requirements. Alabama, Alaska, Colorado, Idaho, Mississippi, Missouri and South Dakota have no statewide licensing procedure. Most other states require some combination of education and training and a clean criminal record. Some states also require schools to submit their curricula and meet specific criteria for state approval. In those states, only people whose education comes from an accredited school can become a licensed investigator.

The length of study and exact steps required to obtain a license vary considerably. In California, applicants must complete specific educational courses and pass a written exam. The terminology can also differ -- in Massachusetts, private detectives hold state licenses, but private investigators do not. Some states require investigators to have liability insurance. Finally, some states allow private investigators to carry firearms. Generally, this requires the investigator to apply for and receive a weapon permit.

Having a license allows a private investigator to practice in one particular state, but the nature of investigative work can require investigators to cross state lines. Some states have reciprocity agreements with one another -- a license in one state allows a person to practice in the other as well. Investigators practicing in states without such agreements sometimes apply for licensure in nearby states as well. Others develop working relationships with investigators in other states, working as assistants, apprentices or trainees when traveling.

While licenses give people the right to present themselves as private investigators, they do not give people the right to break the law in the course of investigations. We'll look at the legal and ethical issues surrounding private investigation in the next section.