Internet of Things (IoT) platforms have multi-layer architectures that facilitate the provisioning, automation of connected devices, and monitoring. These platforms simplify development because they solve a lot of the problems and complexities in building an IoT application. IoT platforms are typically central components with many heterogeneous users, which leads to the effect that these platforms are essential for IoT scenarios and must not become a single point of failure. Furthermore, since many users access these platforms, scalability and multi-tenancy are crucial. The scalability of IoT platforms makes user applications stable and more comfortable to extend. In contrast, the multi-tenancy trait allows multiple tenants to access user applications at the same time. In this thesis, we examine every software stack layer of an IoT platform and explain different methods to make each layer scalable. Enabling multi-tenancy at the application layer and attaining scalability of message brokers, databases, the middleware as a whole, network layer, and device layer are the primary tasks for enabling multi-tenant scalable IoT platforms. To provide a solution to these tasks, we go through the research work already done in the area of scalability and multi-tenancy. We address multiple solutions for each task and also provide the best suitable solution for each task at the corresponding layer of an IoT platform.