Ethernet is a very important communication platform for automation systems. A state-of-the-art industrial Ethernet system is not just a communication protocol but includes a carefully designed communication architecture and a set of communication technologies to be used within this architecture. This chapter explains some of the most important features of modern switched Ethernet networks. Ethernet-based process automation networks have in recent years favored the ring topology due to its simplicity and the abundance of network redundancy protocols for rings. Ethernet switches are multiport devices designed for efficient communication and utilize various filtering mechanisms to optimize the bandwidth utilization. The switch MAC entity receives and transmits Ethernet frames and validates the Ethernet checksum. The concept of Virtual Local Area Networks (VLANs) is realized by tagging each Ethernet frame with a standardized tag. There are two relevant standard protocols for clock synchronization across an Ethernet-based network—Simple Network Time Protocol and Precision Time Protocol.