This Practice Note provides an introduction to the Payment Card Industry Data Security Standards (PCI DSS) for commercial lawyers. It explains the origins of PCI DSS and the Payment Card Industry Security Standards Council (PCI SSC), the entities to which PCI DSS applies, PCI DSS principles and requirements, steps that are required for compliance with PCI DSS, the requirements for ongoing compliance, enforcement, including card scheme fines and interaction with UK privacy and data security laws and standards. Concepts examined include PIN Transaction Security (PTS), Payment Application Data Security Standard (PA-DSS), tokenisation and shared-hosting providers. It also identifies certain key contractual protections that should be sought by merchants from the service providers.