The Platform as a Service (PaaS) scheme within the Cloud Computing paradigm aims to provide a platform for service providers to deploy and host internet-scale applications. It provides the underlying resources and eases their management, provides integration support, data access and authentication as building blocks and orchestration for Service-Oriented Architectures as services are often composites of other services. Two main parts of cloud computing are dynamically scaling resources which adapt to changes in demand and multi-tenancy support to isolate different customers and achieve economy of scale. This thesis takes a multi-tenancy extension to an open source Enterprise Service Bus (ESB) and adds support for horizontal scalability. First two scalability scenarios a examined for pros and cons and possible solutions and their challenges. Then we specify requirements and design and implement a solution with allows the ESB to scale out and add and remove instance based on performance data gathered and distribute incoming request among them.