This book discusses the design and implementation of, as well as experimentation on, an open cross-layer framework and associated methodology to provide voluntary interoperability among heterogeneous Internet of Things (IoT) platforms. It allows readers to effectively and efficiently develop smart IoT applications for various heterogeneous IoT platforms, spanning single and/or multiple application domains. To do so, it provides an interoperable framework architecture for the seamless integration of different IoT architectures present in different application domains. In this regard, interoperability is pursued at various levels: device, network, middleware, services and data.