The Constrained Application Protocol (CoAP) has been designed for RESTful machine-to-machine communication, thereby enabling an Internet of Things. CoAP is based on the principles of the Web, but takes the limited resources of tiny embedded devices such as wireless sensor nodes into account. Despite being relatively new and only about to become an IETF Internet Standard, several implementations of the protocol already exist — each with its own background and supported set of features. In this paper, we give an overview of current CoAP implementations and discuss the results of the first formal interoperability meeting, organized by the European Telecommunications Standards Institute (ETSI) in March 2012. We note that, despite the young age of the protocol, interoperability between the participating implementations is very high, although the non-essential parts of the protocol currently receive significantly less coverage and exhibit slightly more interoperability problems.