The ability to trap an individual ion and cool it close to the ground state of vibrational motion has enabled precise quantum control to be achieved together with very high efficiency measurement. The technology originated over four decades ago in the effort to develop ultra high precision spectroscopy with particular application to frequency standards. In optical ion traps, external lasers are used to couple the internal electronic motion of trapped ions to the vibrational degrees of freedom. This enables a plethora of quantum control protocols, including an implementation of quantum computation, and many analogue quantum simulations of few body quantum systems.