The success story of quantum optics during the past 10 years is largely based on progress in gaining control of systems at the single‐quantum level while suppressing unwanted interactions with the environment, which cause decoherence. Those achievements, illustrated by storage and laser cooling of single trapped ions and atoms and by the manipulation of single photons in cavity quantum electrodynamics, have opened a new field: the engineering of interesting and useful quantum states. In the meantime, the frontier has moved toward building larger composite systems of a few atoms and photons while still maintaining complete quantum control of the individual particles. The new physics to be studied in these systems is based on entangled states and ranges from a fundamental point of testing quantum mechanics for larger and larger systems to possible new applications such as quantum information processing and precision measurements.