Ex-police officers are often particularly attracted to working as private investigators. In general they tend to be people who possess an inquiring mind, an interest in legal matters, and a desire to see justice done. And it is certainly true that skills and experience gained in the police service (not to mention the contacts) can come in very handy for a PI.

Having a police background is by no means essential, however. Anyone, potentially, can become a private investigator. You don't need any special abilities or qualifications (though in some US states and in certain other countries you will have to apply for a licence). A lack of educational qualifications is no barrier to success, and you certainly don't have to be male. Many private investigators are women, who tend to predominate in the matrimonial and domestic areas of the business.

Certain personal characteristics are desirable, though most will come with time and experience. Assertiveness and persistence are important, along with good eyesight (aided by glasses or contact lenses if necessary) and observation skills. And, if you're not the kind of person who relishes sitting in a car all night keeping tabs on a 'suspect', then you are unlikely to make a good investigator.

This is one of very few opportunities where you don't have to be young to succeed. In fact, older investigators are often hired in preference to younger, inexperienced, less streetwise individuals. Maturity and common sense are crucial.

Skills -- physical and acquired -- depend on the kind of work you specialize in. Private eye Irwin Blye has more than thirty years' experience. In his book 'Secrets of a Private Eye' he lists the essential personal characteristics for success as diligence, good observation, being prepared, resourcefulness, creativity, inquisitiveness, patience and neutrality. Fellow author and experienced investigator Edward Smith says the most important traits of all are objectivity, thoroughness, self-reliance and accuracy.

Persistence is a very important attribute. You might spend days tailing a subject, just to find the trail suddenly runs cold. Following this you may go to the public records office to look for written evidence, only to spend several days in a musty vault and find nothing! At this stage a professional investigator takes a deep breath (OK, he might just down a glass of Scotch as well), then decides on his next line of attack. An amateur simply gives up.

Curiosity is another essential attribute without which you'll get nowhere. It's a good idea to bear in mind the six prompts used by the investigative journalist: what, why, when, where, how, who. Every answer to every problem can eventually be found using those six words.

Academically, it helps to have a reasonable command of English for producing reports and other written communications.

Some personal characteristics are simply not compatible with this business. For example, prudishness is rarely a good trait for the investigator who may have to observe and photograph people in compromising positions in order to establish infidelity. And being squeamish won't win you any favors from insurance companies needing you to investigate serious industrial injuries, arson attacks, and people disfigured by road accidents.

One thing you must have is the ability to remain detached. You can't side with one party to a marital disagreement, for example, if you're working for the other party.

Finally, it is a fallacy to suppose that you have to be superfit. OK, a reasonable general level of fitness is desirable, but you certainly don't have to pump iron in the gym every day. Much more important is stamina -- the ability to keep going over long periods without flagging. On a surveillance, for example, you may have to remain alert long past normal working hours.

If you have a reasonable mix of these skills and aptitudes, then congratulations -- you are potentially well suited to working in this fascinating and challenging occupation.