位阻效应（也叫空间效应、空间位阻效应、立体效应）是研究分子中不同基团间电子团重叠形成的电磁力而造成的分子结构或反应取向的立体化学分枝。广泛应用于有机化学中分子结构及反应机理的定性讨论，但在有些情况下可能导致偏差或谬误。

See also: intramolecular forces

Steric effects arise from a fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity.