Wireless Sensor Networks (WSNs) offer unprecedented capabilities for integrating sensing with computing and communication as well as for distributed sensing, coordination and control. While WSNs have been a subject of intensive research for about a decade, most research activities to date focused on sensor nodes typically deployed in static, pre-determined locations with sensor readings taken at regular intervals and multi-hopped to a static sink for subsequent storage and analysis. The next evolutionary step for sensor networks is to handle mobility in all its forms. That is, mobility of sinks, mobility of sensors and actuators as well as mobility of code (i.e. applications). The mobility extension represents a more recent research subject in sensor networking; mobility opens up a whole new level of research opportunities and challenges in WSNs, and significantly expands the types of applications for which WSNs can be used.