The paper presents a set of principles that cover the broad scope of HTA from formulation of the question to effects on the decision-making process. It also provides goals for HTA programs to aim for in producing assessments. The principles are supported by material that includes illustrations from the experience of several programs. The authors have had long contact with the field and are well able to authoritatively discuss a range of methodological and other topics. The paper includes a range of views and details that are relevant to those involved with HTA.