A first network device for use in a wireless sensor network having a plurality of network devices including sensors for monitoring one or more parameters, and a coordinator capable of conducting communications with the network devices over a low-priority link or a high-priority link, wherein the first network device (11 E) is arranged to act as a relay for peer-to-peer communications over a high-priority link or a low-priority link of at least one other said network device, the first network device comprising: recognising means for recognising whether the first network device (11 E) is in an emergency state; and responding means responsive to recognition of the emergency state to establish a high-priority link with the coordinator; characterised in that: the recognising means are further adapted to recognise whether each said other network device is also in the emergency state; and the responding means are further adapted to: for each said other network device which is also in the emergency state, maintain peer-to-peer communications with that network device; for each said other network device which is not in the emergency state, to check whether the first network device has enough resources to support the peer-to-peer communications, and if the first network device has enough resources, to modify the peer-to-peer communications with that other network device, and otherwise to drop the peer-to-peer communications with that other network device. recognising means for recognising whether the first network device (11 E) is in an emergency state; and responding means responsive to recognition of the emergency state to establish a high-priority link with the coordinator; characterised in that: the recognising means are further adapted to recognise whether each said other network device is also in the emergency state; and the responding means are further adapted to: for each said other network device which is also in the emergency state, maintain peer-to-peer communications with that network device; for each said other network device which is not in the emergency state, to check whether the first network device has enough resources to support the peer-to-peer communications, and if the first network device has enough resources, to modify the peer-to-peer communications with that other network device, and otherwise to drop the peer-to-peer communications with that other network device. the recognising means are further adapted to recognise whether each said other network device is also in the emergency state; and the responding means are further adapted to: for each said other network device which is also in the emergency state, maintain peer-to-peer communications with that network device; for each said other network device which is not in the emergency state, to check whether the first network device has enough resources to support the peer-to-peer communications, and if the first network device has enough resources, to modify the peer-to-peer communications with that other network device, and otherwise to drop the peer-to-peer communications with that other network device. for each said other network device which is also in the emergency state, maintain peer-to-peer communications with that network device; for each said other network device which is not in the emergency state, to check whether the first network device has enough resources to support the peer-to-peer communications, and if the first network device has enough resources, to modify the peer-to-peer communications with that other network device, and otherwise to drop the peer-to-peer communications with that other network device.