An investigation about field emission and rf resonant discharges in travelling wave accelerating structures is presented. Resonant electron loading (multipactoring) and non-resonant dark-current are discussed, and starting from a very simple model based on the kinematics of the electrons in rf fields, frequency scaling laws for both multipactoring and dark-currents are found. Our predictions are checked by using the TW-TRAJ code in the case of a LIL-type accelerating structure. Results showing excellent agreement of simulations and experimental measurements up to 100 MV/m of accelerating field are reported. The TW-TRAJ code was extensively used to check the correctness of the frequency scaling laws for the electron loading found by using our naive model in homothetically scaled accelerating structures: the results of our analysis are reported. Last the results of simulations on a CLIC type structure operating at 30 GHz are reported; our analysis shows that no dark current would appear in a CLIC accelerating structure up to an accelerating field of 750 MV/m due to the wide opening of the irises of the accelerating wave guide.