As Grid technology evolves it is becoming evident that the inclusion of mobile devices in a Grid environment will benefit both the Grid and mobile network communities. Many mobile devices, however, have limits on their computational resources that make it difficult to achieve this goal. The Personal Distributed Environment (PDE) is a new concept in mobile networks which, by constraining the problem, overcomes the resource limitation. This paper introduces the concept of the PDE and compares it with the Grid environment. The paper focuses on the specific problem of how to secure the mobile device and in particular how Grid security may be applied to this new environment. The PDE allows simple Grid security mechanisms to be adapted and applied within the PDE boundaries, while allowing a gateway device to support open standards as they emerge. In adopting a hybrid approach, a migration path to open standards is provided, as the computational resources of mobile devices increase.