Our current understanding of the short-time interaction of fast heavy ions with matter is reviewed. Fast ions move along straight line trajectories and loose their energy mainly via collisions with electrons in the close surrounding of the ion path. Thus, the initial stages of corresponding ion tracks are highly excited cylindrical regions with diameters of the order of nanometers that extend over some micrometers in length. The energy dissipation inside these cylinders and the different possible pathways from dense electronic excitations to atomic motions and phase transitions are discussed in the light of recent experimental results.