This Digest describes how thermal mass can be designed into a building - in particular, the options available, and the strategic decisions that may be made during the early design stages that require common understanding of purpose between members of the design team. Although mainly relevant to non-domestic buildings, the information it contains is also appropriate to refurbishment schemes. This first Part (8 pages) defines thermal mass and outlines its effects in a number of scenarios. The second Part (8 pages) discusses the effective incorporation of thermal mass into building designs. A companion document, Information Paper IP 6/01, deals with modelling the performance of thermal mass to minimise annual energy use while maintaining occupant comfort.