Peepholes are useful monitoring devices that make it possible to survey space that is outside of the area currently occupied by the observer. The most common use of the peephole is in the front door of a home or apartment. However, the device is sometimes employed in other applications in the home as well as in the workplace.

The actual construction of a peephole viewer is simple. The device is configured with a small cylindrical body. At each end of the cylinder, a set of lenses help to make it possible to peer through the body and observe activity occurring on the opposite side of the cylinder. When installed in an exterior door, the peephole makes it possible to visually determine who has just rung the bell or knocked on the door before actually opening the door. From this perspective, a peephole can be viewed as a security aid.

Peephole installation is not hard to accomplish. Essentially, a small hole is drilled through the body of the door at a height that is convenient for the homeowner. The door peephole is positioned in the hole and secured on each side by means of a small cover or washer. In some designs, the peephole is incorporated into the design for a decorative door knocker, making the presence of the peephole less conspicuous.

In the workplace, a peephole is sometimes installed to allow discreet monitoring of certain areas of the office. While this function is more commonly handled by security cameras, the peephole has in the past made it possible to monitor who is or is not at their work station, or who is lingering in the break area for long periods. In more clandestine applications, the office peephole may be used to allow office personnel to observe job interviews without the knowledge of the applicant.

Unethical uses of a simple peephole have also occurred in various times and places. When it is determined that the peephole compromised the privacy or rights of an individual, it is often possible to pursue civil or criminal action against the person or persons who made use of the peephole. However, the most common application today is as a safety measure at the front door.