The Payment Card Industry Data Security Standard (PCI DSS) is the mandatory security standard for organizations that handle, process, or transmit payment card data. It defines 12 requirements covering network protection, encryption, access control, continuous monitoring, vulnerability management, and security policies. Compliance requires periodic assessments, remediation activities, and validation via a Qualified Security Assessor (QSA) or Self-Assessment Questionnaire (SAQ) depending on the merchant level.