This document summarizes the key changes to Microsoft Lync Server 2013 architecture and high availability capabilities compared to Lync Server 2010. It discusses topology changes including increased front end capacity and new server roles for archiving and monitoring. It also covers new high availability options for the back end using SQL mirroring, stretched pools for persistent chat, pairing front end pools, and hybrid deployments connecting on-premises and online users. The role of the new Office Web Apps server is also introduced.