蚜虫，又称腻虫、蜜虫，是一类植食性昆虫，包括蚜总科（又称蚜虫总科，学名：Aphidoidea）下的所有成员。目前已经发现的蚜虫总共有10个科约4400种，其中多数属于蚜科。蚜虫也是地球上最具破坏性的害虫之一。 其中大约有250种是对于农林业和园艺业危害严重的害虫。

蚜虫的大小不一，身长从一到十毫米不等。

蚜虫的天敌有瓢虫、食蚜蝇、寄生蜂、食蚜瘿蚊（aphid midge larvae）、蟹蛛、草蛉以及昆虫病原真菌（entomopathogenic fungi，含绿僵菌）。

蚜虫在世界范围内的分布十分广泛，但主要集中于温带地区。另外，物种的多样性在热带比在温带要低得多。蚜虫可以进行远程迁移，主要是通过随风飘荡的形式来进行扩散；例如，莴苣蚜虫被认为就是通过这种方式从新西兰传播到塔斯马尼亚。 而一些人类活动也可以帮助蚜虫的迁移，例如对附着蚜虫的植物进行运输的过程。

Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies, or whiteflies (not to be confused with "jumping plant lice" or true whiteflies), are small sap-sucking insects, and members of the superfamily Aphidoidea. Many species are green but other commonly occurring species may be white and wooly or black. Aphids are among the most destructive insect pests on cultivated plants in temperate regions. The damage they do to plants has made them enemies of farmers and gardeners the world over. From a zoological standpoint they are a highly successful group of organisms. Their success is due in part to the ****ual reproductive capabilities of some species.