Abstract: The advent of laser cooling thirty years ago enabled studies of properties of ultra-cold atomic gases and led to their applications in metrology and quantum information. Against this backdrop, in the last decade a world-wide effort in using ultra-cold atoms as nonlinear media for single photons has emerged. The near-ideal character of interaction between light and atomic ensembles cooled to micro-Kelvin-scale temperatures permits realization of textbook quantum-optical Hamiltonians while coupling to unwanted environments can be nearly eliminated. In this way ultra-cold atoms have been used for generation of single-photon and entangled light fields, their wavelength conversion and entanglement with atoms, and realization of many-body dynamics and long-term storage of quantum states. Besides offering a rich new system for studying quantum mechanics, these advances may find applications in future information distribution and processing systems.