  We present the first laboratory observations of time-resolved electron and
ion velocity distributions in forming, magnetized collisionless shocks. Thomson
scattering of a probe laser beam was used to observe the interaction of a
laser-driven, supersonic piston plasma expanding through a magnetized ambient
plasma. From the Thomson-scattered spectra we measure time-resolved profiles of
electron density, temperature, and ion flow speed, as well as
spatially-resolved magnetic fields from proton radiography. We observe direct
evidence of the sweeping up and acceleration of ambient ions, magnetic field
compression, and the subsequent deformation of the piston ion flow, key steps
in shock formation. Even before the shock has fully formed, we observe strong
density compressions and electron heating associated with the pile up of piston
ions. The results demonstrate that laboratory experiments can probe particle
velocity distributions relevant to collisionless shocks, and thus complement
similar measurements undertaken by spacecraft missions.
