Tailgating is a physical security breach in which an unauthorized person slips through a secured entrance by closely following someone who has legitimate access. Attackers exploit social norms — most people instinctively hold doors open. Effective countermeasures include mantraps or turnstiles, security awareness training, and clear policies requiring employees to challenge unknown individuals. In an ISMS context, tailgating is a physical threat that belongs in your risk assessment. Visitor management procedures and badge requirements further reduce this risk.