Radiant cooling and heating have the potential for improved energy efficiency, demand response, comfort, indoor environmental quality, and architectural design. Many radiant buildings have demonstrated outstanding performance in these regards. However, there are no well-established best practices for design of radiant buildings and their control systems, and most industry professionals are unfamiliar with radiant systems. This study summarizes interviews with eleven professionals with substantial experience with design and operation of radiant buildings in North America. Interviews focused specifically on high thermal mass radiant buildings, referred to as thermally active building systems (TABS). Interviews revealed a diverse range of approaches for design and control of TABS buildings. While interviewees expressed many similar approaches, they also have manyunique preferences. Examples of consistent themes include the use of dedicated outdoor air systems for ventilation and for supplemental cooling, and the use of a relatively simple control schemes that target a constant slab temperature for all times of the day and night. However, interviewees described unique preferences for space types where TABS should be applied, design and types of valves or pumps used for radiant zone control, the control of changeover between slab heating to slab cooling, and many other design considerations. Preferences appear to be driven by project constraints and by personal experience. Interviewees report that their design preferences are effective, but there is no industry consensus about how alternatives compare for energy performance. This paper outlines opportunities for further research, improvement radiant design and control, and the development of best practices.