Health psychologists who undertake consultancy are required to work within the boundaries of ethical practice. However, a standardised framework for how to think about and implement ethical practice in this context has been lacking. Failure to consider ethical issues when conducting consultancy work could lead to inadvertent or iatrogenic harm to individuals affected by the work. It may also result in reputational damage and have financial and legal ramifications. This chapter describes the key principles of ethical practice that health psychologists working in consultancy roles should consider. The principle of respect emphasises valuing dignity and worth, safeguarding privacy, addressing power dynamics, ensuring informed consent, and promoting diversity and inclusion. The principle of competence requires health psychologists to work within their knowledge, skills, and training, ensuring safe and effective practice. The principle of responsibility requires health psychologists to maintain trust, managing their influence appropriately, and prioritising their duty of care to all affected by their work. The principle of integrity requires health psychologists to act honestly, accurately, and consistently, prioritising scientific principles and objectivity while resisting pressures to compromise quality. These principles are defined, and their application to consultancy processes is explored.