Affordability, which implies minimizing both construction and operating costs, central challenge is probably the of architecture today. The twin demands can be met only by simultaneously increasing construction quality and energy efficiency, while keeping its cost down. Innovative, labor- and time-saving construction techniques tend to rely on lighter weight structural components and the utilization of premanufactured components. Under these circumstances, phase change materials (PCM's) offer about the only opportunity to introduce thermal mass into modern buildings. The opportunities discussed here have to do with the relatively light weight of these materials compared to traditional means of providing mass in buildings;and with the flexibility with which these materials may be introduced into structures. The constraints are presented by aesthetic consideration on the one hand, and heat transfer considerations on the other. Ultimately the architectural challenge is to integrate these materials with both the traditional and the emerging techniques to construct buildings and provide them with indoor comfort safely at a reasonable cost.Units: Dual