The article reviews the quantum interface between light and atomic ensembles, a fundamental building block for quantum information processing and state engineering. It discusses various mechanisms such as quantum nondemolition (QND) or Faraday interaction, quantum measurement and feedback, Raman interaction, photon echo, and electromagnetically induced transparency. The paper provides a theoretical framework for these processes, describes experimental techniques and media used, and reviews key experiments on quantum memory for light, entanglement between atomic ensembles and light, and quantum teleportation with atomic ensembles. It also compares homodyne detection and photon counting as measurement techniques and concludes with an outlook on the future of atomic ensembles in quantum information processing.The article reviews the quantum interface between light and atomic ensembles, a fundamental building block for quantum information processing and state engineering. It discusses various mechanisms such as quantum nondemolition (QND) or Faraday interaction, quantum measurement and feedback, Raman interaction, photon echo, and electromagnetically induced transparency. The paper provides a theoretical framework for these processes, describes experimental techniques and media used, and reviews key experiments on quantum memory for light, entanglement between atomic ensembles and light, and quantum teleportation with atomic ensembles. It also compares homodyne detection and photon counting as measurement techniques and concludes with an outlook on the future of atomic ensembles in quantum information processing.