Abstract: Cavity quantum electrodynamics (CQED) is the study of the interaction between light and matter at the single quantum level. It has achieved great success in explaining the strong or weak coupling between light and atoms, quantum coherence, quantum information, and so on. With the local field enhancement effect, micro- and nano-photonic structures can greatly enhance the interaction between light and quantum systems, which brings new opportunities for the study of traditional CQED. Here we review the basic principles, important progress, and possible applications of CQED at the micro/nanoscale, especially the quantum optical effects in metal-based nanophotonic structures. These studies not only enrich our understanding of the interaction between light and matter, but also provide a basis for on-chip quantum information processing and scalable quantum networks.