There is an abundant bibliography on interior design, equipment and furniture. It is possible to detect, in this abundant bibliography, procedural constants when theorists deal with this topic: once a theme is introduced (materials, shape, color, history of styles, to name just a few examples), and after giving a succinct definition of the interior spaces, the theory is immediately abandoned and the subject in question is illustrated with graphic examples. The examples cited are presented as paradigmatic cases, assuming that by observing them, you can learn to design interior spaces and furniture. Most likely, in these approaches to interior design, equipment and furniture, the idea that this type of design (IDEaF) is a less complicated activity than architecture, whose problems can be addressed through recipes or tips that guarantee good compositions formal, colored or stylish. It is necessary to establish some criteria to begin to do theory where references to fashions or styles do not prevail. And the way to start doing theory is defining ourselves basic concepts of this practice, which is interior design, equipment and furniture, in order to create solid foundations from which to carry out a critical activity.