An artificial atom is an object that has bound, discrete electronic states, as is the case with naturally occurring atoms or molecules. In an artificial atom the charge and energy of a sufficiently small particle of metal or semiconductor are quantized just like those of an atom. Semiconductor quantum dots are the most common example of artificial atoms, and are analogies for real atoms. The current through such a quantum dot or one electron transistor reveals atom-like features in a spectacular way. In the form of quantum dots they are made up of tens to thousands of atoms. They are like single atoms in one important way: when provided with the right amount (or quanta) of energy, they will absorb that energy and may give it off as coloured light.