Attosecond pulses are generated by electrons that are extracted from an atom, molecule or solid by an intense light pulse and then travel under the influence of the electric field of the light.  Portions of each electron wave packet are forced to re-collide with its parent ion (or hole) after the field reverses direction.   Upon re-collision, the electron can recombine, emitting soft X-ray radiation that can be in the form of attosecond pulses.  This highly nonlinear process offers unique measurement opportunities – for measuring the attosecond pulse itself; the orbital(s) from which it emerged; and the band structure of material in which the wave packets moved.