Correlated electrons in transition metal oxides can form a variety of electronic phases. The phase change between these various states gives rise to novel device functions, including sensing, signal conversion, and nonvolatile memory, and is now at the frontier of research on "emergent research device materials." Those oxide devices may have an advantage over conventional semiconductor devices for added functionality and future downsizing to the nanoscale. The elucidation of the microscopic physics behind their operation is a key step for further development.