Presented here is the philosophical foundations and practical applications of principlism in AI ethics. It discusses core ethical principles like beneficence, autonomy, justice, and explainability, critically assessing their effectiveness and limitations when operationalised in real-world settings. The chapter highlights the gap between ethical theory and practice, examining issues of enforceability, corporate co-option, and interpretative divergences. It argues for an institutional embedding of ethics, recommending an active role for AI ethics advisors to navigate competing interests and to ensure that ethical reasoning informs policy and practice.