During its lifetime, ArcGIS for Server has proven resilient to the advances of technologies. It managed to adopt and evolve as more users started migrating to the technology despite the limitations of its architecture at that time. With 10.1, Esri has introduced a completely new architecture. This enabled the product to consume resources more efficiently and increase the execution throughput effectively utilizing the 64-bit instruction set and hyper multithreading. Upgrading from the previous architecture didn't come free of pain to Server users. The users, especially the developers, had to rewrite their websites along with any libraries they developed if they wanted to migrate to the new version. In this appendix, we will explain the difference between the previous and the new architectures, emphasizing on the benefits of the new architecture.