In information security and privacy, personally identifiable information or personally identifying information (PII) is any piece of information which can potentially be used to uniquely identify, contact, or locate a single person.

Although the concept of PII is ancient, it has become much more important as information technology and the Internet have made it easier to collect PII, leading to a profitable market in collecting and reselling PII. PII can also be exploited by criminals to stalk or steal the identity of a person, or to plan a person's murder or robbery, among other crimes. As a response to these threats, many web site privacy policies specifically address the collection of PII, and lawmakers have enacted a series of legislation to limit the distribution and accessibility of PII.

Items which might be considered PII include, but are not limited to, a person's:

• Full name (if not common)
• National identification number
• Telephone number
• Street address
• E-mail address
• IP address (in some cases)
• Vehicle registration plate number
• Driver's license number
• Face, fingerprints, or handwriting
• Credit card numbers
• Digital identity

Information that is not generally considered personally identifiable, because many people share the same trait, include:

• First or last name, if common
• Country, state, or city of residence
• Age, especially if non-specific
• Gender or race
• Name of the school they attend or workplace
• Grades, salary, or job position
• Criminal record

When a person wishes to remain anonymous, descriptions of them will often employ several of the above, such as "a 34-year-old black man who works at Target". Note that information can still be private, in the sense that a person may not wish for it to become publicly known, without being personally identifiable. Moreover, sometimes multiple pieces of information, none of which are PII, may uniquely identify a person when brought together; this is one reason that multiple pieces of evidence are usually presented at criminal trials. For example, there may be only one Inuit person named Steve in the town of Lincoln Park, Michigan. In forensics, the tracking down of the identity of a criminal, personally identifiable information is critical in zeroing in on the subject. Criminals will go to great trouble to avoid leaving any PII; they wear masks (faces and hair are PII), gloves (fingerprints are PII), clothing that covers personal marks (tattoos and scars are PII) and avoid writing anything in their own handwriting (handwriting can be PII). Also, more modern 'masks' may be used, such as using a proxy IP address to avoid being tracked online as easily. In some professions, it is dangerous for a person's identity to become known, because this information might be exploited violently by their enemies; for example, their enemies might hunt them down or kidnap loved ones to force them to cooperate. For this reason, the United States Department of Defense (DoD) has strict policies controlling release of PII of DoD personnel. This is also the reason usually given in fiction for superheros and secret agents to disguise their faces and withhold their real names.