This paper presents a survey of existing techniques for achieving mobile code security, as well as a representative sampling of systems which use them. In particular, the problem domain is divided into two portions: protecting hosts from malicious code; and protecting mobile code from malicious hosts. The discussion of the malicious code problem includes a more in-depth study of the Java security model, as well as touching upon several other systems. The malicious host problem, however, is much more difficult to solve, so our discussion is mostly restricted to ongoing research in that area.