**Peeking into quantum black boxes**

Assembling a quantum information processing circuit requires precise knowledge of the properties of its each component, i.e. the ability to predict the effect of the component on an arbitrary input quantum state. This knowledge is acquired through quantum process tomography - a procedure in which certain \"probe\" states are sent into the quantum \"black box\", and the corresponding outputs are measured. Surprisingly, black boxes that process quantum optical information can be completely characterized using coherent states, i.e. simple laser pulses, as the probe states. We shall discuss the theory and implementation of this procedure as well as a few examples of its practical application.