This chapter explores how health technology assessment (HTA) has evolved and the role of patient involvement in it. HTA seeks to inform decision-makers about the value of a health technology through a systematic process that assesses the best available evidence about all its impacts, compared with the current standard of care. This requires interdisciplinary deliberative discussion and value judgements about the relevance of the evidence for the local health system. HTAs may lead to recommendations about the use or disinvestment of a health technology and so are subject to political, public and stakeholder scrutiny. As a result, there has been pressure to involve those who have a specific interest in the health technology, particularly patients, in the HTA process. This has been considered as contentious by some, due to concerns about potential bias, representativeness of patient input and the scientific integrity of research into patient aspects. However as rigorous processes for patient involvement in HTA have been developed and evolved, the value of patient involvement in various steps of the HTA process have been recognised by decision-makers. Consequently, patient involvement in HTA is now seen by many as a necessity on democratic, ethical and research grounds.