How non-adiabatic are surface dynamical processes?

The state of experiments quantifying the significance of non-adiabatic effects in dynamical processes at metal surfaces is reviewed. It is discussed to which extent these effects can be explained in a unifying view. Deficiencies in the present understanding and modelling of chemical reaction dynamics at metal surfaces are pointed out. Lines connecting the recent experimental and theoretical work dealing with external and internal exoelectron emission, abstractive chemisorption, possible non-adiabatic effects in gas-surface dynamics and vibrational energy transfer are discussed.