In many ways, a private investigator is like a police officer, sheriff's deputy, or a police detective (all referred to as "peace officers"). As such, the primary function of a P.I. is to undertake an investigation. And like peace officers, a private investigator may investigate a whole range of situations.

There is one major difference between a private investigator and a peace officer, however. A private investigator is someone who conducts investigations as a private citizen or as the member of a private firm, whereas a peace officer is a public servant employed by a government agency. In criminal matters, a peace officer's job is reactive, meaning they react to a crime that has already been committed. At that time the peace officer investigates the circumstances of the crime from the state's perspective in order to identify and prosecute the offender (perpetrator). The private investigator usually works for the defense attorney representing the interests of the person who's been accused of the crime.

In civil matters, a private investigator's position could be viewed as either reactive or proactive. When investigating a case where a wrong has already been committed, the PI is reactive, gathering data to support his client's position. When gathering information before a wrong has been committed (doing background checks, searching for assets, fiding missing people, many family situations), the PI acts proactively to prevent a wrong from being committed, much like a security guard who acts proactively to prevent shoplifting or the wrongful entry onto private property.

Private investigators are also called private detectives, PIs, or private eyes. More recently, however, private investigators have used the professional title, ‘professional investigator’ or even 'legal investigator'. Primarily, however, the change in title is an effort to negate the somewhat ‘seedy’ image that has been attributed to private investigators... courtesy of Hollywood and many fiction writers.