When is the 'right' time to initiate an assessment of a health technology?

There is currently no generally accepted formula for the optimal timing of health technology assessments (HTAs). This paper presents some of the relevant issues and then reviews the existing literature on timing of HTAs. It finds that the literature that specifically addresses these issues is limited. There is a consensus that HTAs should be initiated at an early stage of the development of a new health technology, and repeated during the life cycle of the technology. However, the questions of reliably identifying new technologies at an early stage in their development and of deciding on a detectable critical point for starting evaluation are not resolved. It is proposed that a system of categorization and prioritization of health technologies should be developed to allow decisions to be made as to when a strongly precautionary approach is required and how the limited resources available for HTA could be optimally deployed.