Current focus of sensor networks is on systems dedicated for a specific application. Wide variety of circumstances point to the need for sensor network deployments capable of sharing the physical network resources. We introduce the concept of Virtual Sensor Networks (VSN) to provide protocol support for the formation, usage, adaptation and maintenance of subsets of sensors collaborating on specific tasks. VSN concept also promises to enhance applications that involve dynamically varying subsets of sensor nodes collaborating tightly to achieve the desired outcomes, while relying on the remaining nodes to achieve connectivity and overcome the deployment and resource constraints. This article explores the concept, mechanisms, and benefits of using VSN. Examples of VSN support functionality include support for nodes to join and leave VSNs, broadcast within a VSN, and merging of VSNs. VSNs will simplify application deployment, enhance performance and scalability, facilitate resource sharing, and provide a degree of physical topology independence in wireless sensor networks. Deployment of overlapping sensor networks aimed at different tasks, especially in harsh environments, will also significantly benefit when organized as VSNs over a shared infrastructure.