The idea of improving the thermal comfort of lightweight buildings by integrating phase-change materials (PCMs) into the building structure has been investigated in various research projects over several decades. Most of these attempts applied macro-capsules or direct immersion processes, which both turned out to present several drawbacks. Due to these problems, none of these PCM products was successful in the wider market. The new option to micro-encapsulate PCMs, a key technology which overcomes many of these problems, may make PCM products accessible for the building industry. This paper describes the work done at Fraunhofer ISE within a German government-funded project over the last 5 years, extending from building simulations to first measurements of full-size rooms equipped with PCM. The first products are now available on the market. (C) 2005 Elsevier B.V. All rights reserved.