An attempt is made in this paper to systematize the ideas behind low-energy building concepts and energy-efficient building solutions. The background to this is that many low-energy building concepts seem to have as a goal ＇the exclusion of external purchased energy＇ when energy-efficient solutions strive towards ＇the lowest possible energy requirements with reasonable utilization of resources＇. However, in discussions about research and demonstration projects in this field and in reports published from these projects, this distinction is seldom, if ever, made. Especially when it is a question of low-energy buildings, technical solutions are often suggested and demonstration projects are carried out without a clear analysis of the applicability in practice. This is reflected in several books and reports which describe different low-energy projects thoroughly without providing the information needed for an unbiased assessment of the applicability on a greater scale in unsponsored applications. The importance of defining the goals of, and the basic ideas behind, low-energy projects is emphasized and a system for accomplishing this is introduced. It is emphasized that solutions combining low-heat-loss building design with advanced heat supply systems should be treated with caution. It is also pointed out that there are basic differences between residential buildings and commercial buildings and it is vital to be aware of these differences when discussing low-energy buildings or energy-efficient buildings.