This thesis, entitled Wireless Sensor Networks in Hostile RF Environments, was submitted to the The University of Manchester by Mr Dominic James Patrick Crutchley on 30th April 2012 for the degree Doctor of Philosophy (PhD). This thesis considers two different but related aspects of wireless communication in Wireless Sensor Networks (WSNs) operating in hostile environments.Using grain as an example of a hostile environment, the influence of hostile, attenuating media on Radio Frequency (RF) communications was considered. Further to this the implications of a hostile environment for protocol stacks were considered, and a cross-layer, cross-application framework was proposed to help future protocol designers address these issues.To achieve both these aims, the software for a bespoke WSN node was designed and implemented. The node was characterised to ensure a good understanding of its RF behaviour and practical experiments were then conducted in a small-scale grain silo to gain an understanding of attenuation and data communications within grain.Finally, a real world implementation of the proposed cross-layer, cross-application framework was produced and a small example cross-layer protocol was demonstrated running on the WSN node.It was shown that a WSN can be used to characterise communications within a hostile medium and also that data communications are achievable within grain. It was also shown that a small cross-layer, cross-application framework could ease the development of cross-layered protocols in WSN software.