Mobile Agents have promised much as an Internet programming technology, but well publicized security problems are hampering their acceptance. This paper argues that mobile agents are a vital component of any Internet security solution. For instance, agents can move computations to safe areas, and can replace out-dated security mechanisms that adapt poorly to the Internet. To underline this argument, the paper proposes a security architecture that uses mobile agents as its basic mechanism. The semantics of agent security policies in this architecture is also given.