This Practice Note explains how the principles of legal professional privilege (LPP) apply in a criminal context. It explains the different categories of legal professional privilege, legal advice privilege and litigation privilege, and how these key concepts apply in criminal investigations and proceedings. This Practice Note also considers the impact of the decisions in SFO v ENRC, HSE v Jukes and R (on the application of Jet2.Com Ltd) v CAA on the dominant purpose test in litigation privilege and legal advice privilege and what constitutes ‘reasonable prospect of litigation’ in litigation privilege in criminal investigations. It also considers the difference in the protections afforded by legal advice privilege as opposed to litigation privilege in criminal proceedings.