The concept of a streak camera operating throughout the far-infrared (1-100µm) is discussed. The photocathode, used in a conventional streak camera to create an electron pulse, is replaced by gas phase atoms in a highly excited state, a Rydberg state. Rydberg electrons have an extremely low binding energy (work function) combined with a high FIR photo-ionization cross section. A streak camera based on an atomic-vapour photocathode is demonstrated in the visible and a sub-picosecond resolution is obtained.