Much of this book has focused on photons, electrons, and solids. But atomic and ionic systems are also of great interest in modern applications of quantum mechanics. Atom interferometry is a well-developed field that is useful for fundamental tests of quantum physics. Atomic and ionic systems allow the implementation of highly stable qubits and the manipulation of those qubits by means of electromagnetic pulses and so are a strong candidate for the implementation of quantum computers, as well as being suitable for many high-precision sensing applications. The atoms and ions are usually used in the form of dilute gases that need to be cooled to low temperatures. So, in the following sections, we first look at methods for trapping and cooling atoms and ions. We then look at how atom-photon interactions can be used to form and process well-defined qubits.