Tailgating is a social engineering act that occurs when an unauthorized user follows an authorized user into a system without the authorized user's consent. An example of tailgating is when a user (the victim) leaves their computer for let's say coffee, lunch, or meeting, and another user (the malicious actor) goes onto their unlocked computer. Tailgating most commonly happens when a user leaves their computer without locking it due to negligence, forgetfulness, or even laziness.

Reasons for tailgating can range from malicious activity/corporate espionage to personal attacks on other employees. Either way, tailgating by insider threats can cause serious security breaches and anti-tailgating measures should always be put in place. Although tailgating is against most organizations' policies, many users tend to overlook this. 

In digital security, tailgating happens when an unauthorized user gains access to another user's computer and/or website access. In physical security, tailgating generally occurs at access control points such as doors and turnstiles.