Often, security officers are uniformed and act to protect property by maintaining a high visibility presence to deter illegal and inappropriate actions, observing (either directly, through patrols, or by watching alarm systems or video cameras) for signs of crime, fire or disorder; then taking action and reporting any incidents to their client and emergency services as appropriate.

Since at least the Middle Ages in Europe, the term watchman was more commonly applied to this function. This term was carried over to North America where it was interchangeable with night-watchman until both terms were replaced with the modern security-based titles.

Functions and duties

Many security firms and proprietary security departments practice the "detect, deter, observe and report" methodology. Security officers are not required to make arrests, but have the authority to make a citizen's arrest, or otherwise act as an agent of law enforcement at the request of a police officer, sheriff, and others.

In addition to the methodology mentioned above, a private security officer's primary duty is the prevention and deterrence of crime. Security personnel enforce company rules and can act to protect lives and property. In fact, they frequently have a contractual obligation to provide these actions. Security officers are often trained to perform arrest and control procedures (including handcuffing and restraints), operate emergency equipment, perform first aid, CPR, take accurate notes, write detailed reports, and perform other tasks as required by the contractee they are serving.

Many security officers are required to go through additional training mandated by the state for the carrying of weapons such as batons, firearms, and pepper spray (e.g. the Bureau of Security and Investigative Services1 in California has requirements that a license for each item listed must be carried while on duty). Some officers are required to complete police certification for special duties.

Positions are also set to grow in the U.S., with 350,000 new security jobs expected over the next decade. In recent years, due to elevated threats of terrorism, most security officers are required to have bomb-threat training and/or emergency crisis training, especially those located in soft target areas such as shopping malls, schools, and any other area where the general public congregate.

One major economic justification for security personnel is that insurance companies (particularly fire insurance carriers) will give substantial rate discounts to sites which have a 24-hour presence; for a high risk or high value venue, the discount can often exceed the money being spent on its security program.

This is because having security on site increases the odds that any fire will be noticed and reported to the local fire department before a total loss occurs. Also, the presence of security personnel (particularly in combination with effective security procedures) tends to diminish "shrinkage," theft, employee misconduct and safety rule violations, property damage, or even sabotage. Many casinos hire security guards to protect money when transferring it from the casino to the casino's bank.