Reliable, scalable and secure messaging is the fundamental building block to the Enterprise Service Bus (ESB) and how it provides the ability to serve as the transport layer to enterprise grade applications. The focus of this document is to describe the key components of the messaging/transport layer and the concepts involved as a building block to understanding the whole ESB. While all of these concepts and features are available using the SonicMQ JMS API, this discussion is purely to reference the configuration within the ESB as illustration. A deeper discussion of JMS could easily take 100s of pages to describe all the concepts and techniques and is beyond the scope of this paper. What is included is the most common concepts and techniques used as part of an ESB deployment, some of which are extensions to the core JMS specification.