Transaction-level modeling --- originally used decades ago in the development of telecommunications network architecture --- is now widely used in SoC design. Why? Because the modern SoC is now so complex that systematic modeling and analysis are required to devise the optimal chip architecture. The architectural model is the essential platform that kick-starts two other key tasks --- verification testbench development and software development. In addition, the interoperability imperatives of SoC design and verification, IP reuse, software development, and system evaluation and integration have driven the replacement of proprietary transaction-level modeling methodologies by a TLM standard that leverages the power of SystemC. The standard --- devised by the Open SystemC Initiative (OSCI) in collaboration with the Open Core Protocol International Partnership (OCP-IP) --- covers the multiple levels of abstraction required for all of the foregoing tasks.