Mobile Applications are programs which are able to move themselves between hosts on the network. Sentient Applications are programs which can exploit the existence of pervasive networked sensor devices to observe their environment and react accordingly. We believe that properly designed and constrained Sentient Mobile Applications provide a good foundation for building applications for pervasive computing environments. The aims of this work are threefold: (i) motivate the use of Sentient Mobile Applications in next-generation pervasive computing environments; (ii) describe the role of policy in building Sentient Mobile Applications; (iii) demonstrate the need for policy to control Sentient Mobile Applications once they have been deployed.