边际主义在经济学中指由边际效用和边际成本（参看：边际）发展而来的经济学理论。边际效用理论是在1870年左右，是由英格兰的威廉姆·斯坦利·杰文斯、奥地利的卡尔·门格尔和瑞士的利昂·瓦尔拉斯各自独立发展起来的，他们最终使边际主义广泛流传，但是赫尔曼·海因里希·戈森（HH Gossen）早就指出了交换价值和边际效用之间的关系。经济学思想的这次变革被称作新古典主义革命或边际革命。

边际主义的指出了在决策中最重要是边际，即最后一单位的消费品或是产品。例如，有一辆车可以方便出行，假如它要维修或作为备用车，第二辆车也相当有用，但不如第一辆车有用，第三辆车就更加不如前面的两辆车了。给定汽车的价格，没有人会相信会有人去购买三辆车，因为第三辆车的收益可能低于其价格。

Marginalism is a theory of economics that attempts to explain the discrepancy in the value of goods and services by reference to their secondary, or marginal, utility. The reason why the price of diamonds is higher than that of water, for example, owes to the greater additional satisfaction of the diamonds over the water. Thus, while the water has greater total utility, the diamond has greater marginal utility. The theory has been used in order to explain the difference in wages among essential and non-essential services, such as why the wages of an air-conditioner repairman exceed those of a childcare worker.