WSNs have become pervasive and are used in many applications and services. Usually, deployments of WSNs are task-oriented and domain-specific, thereby precluding reuse when other applications and services are contemplated. This inevitably leads to the proliferation of redundant WSN deployments. Virtualization is a technology that can aid in tackling this issue, as it enables the sharing of resources/infrastructure by multiple independent entities. In this article we critically review the state of the art and propose a novel architecture for WSN virtualization. The proposed architecture has four layers (physical layer, virtual sensor layer, virtual sensor access layer, and overlay layer) and relies on a constrained application protocol. We illustrate its potential by using it in a scenario where a single WSN is shared by multiple applications, one of which is a fire monitoring application. We present the proof-of-concept prototype we have built along with the performance measurements, and discuss future research directions