Atomic frequency standards are used to generate accurate and precise time and frequency, enabling many communications, synchronization, and navigation systems in modern life. GPS and other satellite navigation systems, voice and data telecommunications, and timestamping of financial transactions all rely on precise time and frequency enabled by atomic frequency standards. This review provides a snapshot and an outlook of commercial and emerging atomic frequency standards. We provide a concise summary of the performance and physics of operation of current atomic frequency standards. In addition, we discuss examples of emerging frequency standard technologies and prototype demonstrations with a focus on technologies expected to provide commercial or military utility within the next decade. We include a comparison of performance versus size and power for current atomic frequency standards. We develop and discuss an empirical relationship between frequency standard performance and product size.