运载体是基因工程中将DNA片段（常为目的基因）转移至受体细胞的运载工具。运载体包含或本身就是是特殊的DNA片段，进入活细胞后有些运载体甚至能自我复制，运载体的基因组即便常不在宿主细胞的基因组内，仍然可以得到表达。通过将外源DNA插入载体DNA并导入到活细胞中，就可以在活细胞中表达外源DNA，而进一步研究加载的DNA片段，其表达的RNA或蛋白质，或被表达所影响的特定途径。常用的运载体有细菌质粒、噬菌体和动物病毒或植物病毒。

In molecular cloning, a vector is a DNA molecule used as a vehicle to artificially carry foreign genetic material into another cell, where it can be replicated and/or expressed. A vector containing foreign DNA is termed recombinant DNA. The four major types of vectors are plasmids, viral vectors, cosmids, and artificial chromosomes. Of these, the most commonly used vectors are plasmids. Common to all engineered vectors are an origin of replication, a multicloning site, and a selectable marker.