A police officer is a person who works for a police force. It usually only refers to those who have been sworn in as law enforcement officers, and does not include civilian support personnel (some of whom may be uniformed and have certain limited enforcement powers).

A police officer (also known as a constable in some countries) is employed in most cases by federal, state/provincial or municipal governments and has the responsibility (or duty) of enforcing federal, state/provincial laws along with municipal/city ordinances. They also have the responsibility of keeping the public peace. This is usually done by uniformed pro-active patrolling within their jurisdiction looking for and investigating law breakers, and by responding to calls for service. Police officers are required to keep notes of all situations in which they take action and appear as witnesses during both criminal prosecutions and civil litigation. One of the lesser-known but most time-consuming duties of officers is completion of documentation of activity ("reporting").

It must be noted that the responsibilities of a police officer/constable are extremely broad and not in any way limited to the duties mentioned above. Police are expected to be able to respond in some fashion to any and all situations that may arise while they are on duty. In some communities rules and procedures governing conduct and duties of police officers requires that they act if needed even when off duty.

In most countries, candidates for the police force must have completed some formal education. Increasing numbers of people are joining the police force who possess tertiary education and in response to this many police forces have developed a "fast-track" scheme whereby those with university degrees spend 2-3 years as a police constable before receiving promotion to higher ranks, such as plain clothes detective. Police officers are also recruited from those with experience in the military or security services. Most law enforcement agencies now have measurable physical fitness requirements for officers. In the United States, state laws codify state-wide qualification standards regarding age, education, criminal record, and training.

Police agencies are usually semi-military in organization, so that with specified experience or training qualifications officers become eligible for promotion to a higher supervisory rank, such as sergeant. Promotion is not automatic and usually requires the candidate to pass some kind of examination, interview board or other selection procedure. Although promotion normally includes an increase in salary, it also brings with it an increase in responsibility and for most, an increase in administrative paperwork.

After completing a certain period of service, officers may also apply for specialist positions, such as detective, police dog handler, mounted police officer, motorcycle cop, water police officer, or firearms officer (in forces which are not routinely armed).

In addition to any formal qualifications required, potential police officers should have a genuine interest in working with the public and possess an inquiring mind.

Police officers face many dangers in their jobs. Police officers are constantly being faced with the unknown and the unpredictable. They never truly know the outcome of any situation they enter into. This can make policing a dangerous profession. Dangers faced by police include death, increased risk of infectious diseases, and serious and minor trauma, both physical and emotional. These dangers are encountered in many different situations i.e. apprehension/arrest and investigation of criminals, conducting vehicle stops, investigating people and crimes, protecting the public from dangerous situations or individuals, investigating traffic accidents and witnessing the carnage that often results from those accidents, responding to suicides and directing traffic.

However, many police officers do derive a genuine sense of job satisfaction from their work, especially when a long investigation has been successfully completed. On a day-to-day basis, officers can often see the results of their work in their ability to help members of the public, even with incidents of relatively minor importance.

In the United States, 18,092 law enforcement officers are known to have died in the line of duty. In Canada, 757 law enforcement officers are known to have died in the line of duty. In the United Kingdom, about 3,600 law enforcement officers are known to have died in the line of duty.

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