The Internet of Things (IoT) reshaped our world by interconnecting and empowering real objects with computational capabilities to build smarter solutions. By nature, the IoT domain is deeply heterogeneous, encompassing many devices, technologies, and protocols. Several IoT platforms have been proposed to support and lower the barriers to developing IoT applications. However, selecting a specific platform can lead to vendor lock-in problems since the deployed application strongly depends on the platform’s functionalities. This lack of portability is considered an open challenge in both industry and research fields. Real-world examples, such as the discontinuation of well-known IoT platforms, further exacerbate this problem. Indeed, migrating IoT applications from one platform to another often requires significant adaptations or a complete redesign. To address portability across different IoT platforms, this chapter presents X-IoT (read as cross-IoT), a model-driven modeling method supporting the development and multiple deployments of cross-platform IoT applications. We examined many IoT platforms and extracted their functionalities to construct a Domain-Specific Modeling Language (DSML) with a graphical notation that represents the elements of an IoT application relying on an IoT platform. These elements permit modeling IoT application structure, logic, and data visualization. The homonym tool developed employing the ADOxx metamodeling platform supports the methodology. This tool allows modeling platform-independent IoT applications that can be successively refined and deployed on multiple IoT platforms. A fleet tracking scenario demonstrates the X-IoT’s applicability.