Any horizontally oriented structural member that primarily carries vertical normals (loads caused by gravity), and secondarily lateral loads (winds, earthquakes, etc). Beams rest on columns, walls, girders, and in some instances grade (see grade beam), which then transfer the loads they carry to other members, eventually dissipating at grade.

Beams can be of any tensile material, such as metal or wood. When cast out of concrete, it is typically necessary to reinforce the beam; otherwise the beam must be prestressed. The most common type of beam is the wide flange section, commonly mistaken for an I-beam.

Beams, like many other structural members, are placed under multiple stresses within itself; the top of the beam will experience compression, while the bottom will experience tension. Somewhere in between, commonly in the center for equally symmetric members, the neutral axis will be present, which is the axis upon which all forces equal zero.