防御工事泛指一切军事上增加防守方优势的临时或永久建筑，例如要塞、城墙、壕沟、城堡、障碍物...等。

防御工事自古就是战争中防守方有形优势的主要来源，因为只有防守方能拥有，而攻击方无法拥有，以逸待劳是防御工事的存在条件之一，因此建筑地点必须是假想战争中敌军可能进攻或经过的路在线，防御工事的劣势在于无法便利的搬动（或是无法移动）所以若是战略构想错误，建筑在错误位置则毫无用处，例如法国的马奇诺防线因为被绕过而未发挥效益。

Fortifications are military constructions or buildings designed for the defense of territories in warfare, and also used to solidify rule in a region during peace time. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs. The term is derived from the Latin fortis ("strong") and facere ("to make").