This article describes the basic principles of evaluation, focusing on the evaluation of healthcare services. It emphasises the importance of evaluation in the current healthcare environment and the requirement for nurses to understand the essential principles of evaluation. Evaluation is defined in contrast to audit and research, and the main theoretical approaches to evaluation are outlined, providing insights into the different types of evaluation that may be undertaken. The essential features of preparing for an evaluation are considered, and guidance provided on working ethically in the NHS. It is important to involve patients and the public in evaluation activity, offering essential guidance and principles of best practice. The authors discuss the main challenges of undertaking evaluations and offer recommendations to address these, drawing on their experience as evaluators.