We present a security framework for executing foreign programs, called helpers, on a Pocket Companion: a wireless hand-held computer. A helper program as proposed in this paper is a service program that can migrate once from a server to a Pocket Companion or vice-versa. A helper program is convenient, provides environment awareness and allows asynchronous interaction. Moreover helpers can be used to save processing power and to reduce communication. By migrating to the location of a resource, a helper can access the resource more efficiently. This is particularly attractive for mobile computing, where the network conditions can be poor and unreliable, and because it does not require a permanent connectivity. Security is a significant concern for helpers, as the user of a Pocket Companion receiving a piece of code for execution may require strong assurances about the helper's behaviour. The best way to achieve a high security is to use a combination of several methods. We are designing a prototype of a helper system, called Harpoon, on top of the Inferno operating system.