The controversy about experimental properties of cathode spots is reviewedwith special consideration of clean surfaces. From arc tracks in vacuum itis concluded that the density of the emitted electron current is as high as10l2 A/m2. It is demonstrated the spots are very non stationary with basictime constants of the order of nanoseconds, in spite of the thermal inertiaof the cathode metal. This can be explained by models of convective heattransport. It is suggested that much more metal is molten than finally removed, so the erosion rate is not a simple measure of surface heating phenomena. Ignition of spots by plasmas is always due to an interaction withsurface contaminants. This is also the reason for conditioning phenomenawith arcs in fusion devices. In the presence of strong magnetic fields thelatter exhibit an additional voltage drop outside the spot. Therefore theyare possible only as transient events during instabilities of the fusionplasma.