A beam is a structural element that carries load primarily in bending (flexure). Beams generally carry vertical gravitational forces but can also be used to carry horizontal loads (i.e. loads due to a gust of wind or an earthquake). The loads carried by a beam are transferred to columns, walls or girders , which in turn transfer the force to adjacent structural members.

Beams are characterized by their profile (the shape of their cross-section), their length, and their material. In contemporary construction, beams are typically made of steel, reinforced concrete, or wood. One of the most common types of steel beam is the I-beam or wide-flange beam (also known as the "universal beam"), commonly used in steel-frame buildings and bridges.

Internally, beams experience both compressive and tensile stresses as a result of the loads applied to them. Under gravity loads, the top of the beam is under compression while the bottom of the beam is under tension, leaving the middle of the beam relatively stress-free. The I-beam is so common because it makes efficient use of material for carrying loads in bending—it maximizes material at the top and bottom of the beam where the bulk of the load is carried.

Mathematical methods for determining the deflection of beams include "method of virtual work" and the "slope deflection method." Engineers may be interested in determining deflections because the soffit of a beam may be in direct contact with a brittle material such as a glass panel. A stiffer beam (high Young's modulus and high Moment of inertia) produces less deflection. Mathematical methods for determining the beam forces (internal forces of the beam and the forces that are imposed on the beam support) include the "moment distribution method," the "force or flexibility method," and the "stiffness method."