Mobility creates new challenges for dynamic systems, which need a new conceptional treatment: systems, that deal for example with mobile agents, need extended security concepts to handle the risks, induced by foreign, untrusted agents. In this work we present the paradigm of nets within nets, which is well suited to express the dynamics of open, mobile systems. It is furthermore capable of formalising security aspects and analysing the correctness of a given system in terms of the security specification. This approach is attractive since it combines a modelling-oriented view with an exact formalism.