In the United States and internationally, the trend for public engagement in health policy and practice is increasing, particularly regarding health technology assessment (HTA), which informs often controversial coverage decisions. However, there is no consensus about which members of the public should be involved in which processes or what the respective rationales and benefits of public engagement are. This article explores operational processes and underlying rationales of public engagement at HTA agencies in France, Germany, and the United Kingdom. The analysis is based on website information, legal framework documents, published and gray literature, and semistructured, in-depth interviews with top officials at these agencies. Engagement processes differ across agencies, particularly regarding the areas in which the public is involved, which groups of the public are involved, what weight they have in influencing decisions, how they are recruited and supported, and how potential conflicts of interests are addressed. Different emphases on rationales and drivers behind public engagement partly reflect the respective political environments. Interviewees indicated a range of benefits of engagement and factors influencing success or failure. The results highlight the need to be clear about the purpose and conduct of engagement in order to maximize the benefits of this increasingly widespread policy tool.