The well established practice of applying tip and/or root relief to the teeth of low contact ratio spur gears is reviewed. Results are presented for the experimental validation of a systematic design method for profile relief that depends critically on how far the relief extends along the path of contact. This method has proved to be effective in controlling gear vibration and noise. A good correlation between static transmission error, dynamic transmission error and sound pressure level was found. This allows design rules to be formulated which can be applied to achieve the minimum vibration and noise levels at a given operating load. A computer program was also developed that allows the introduction of measured gear tooth profiles from an involute tester, to predict the static transmission error curves and thus give an indication of gear quality as regards vibration and noise levels for any given simulated operating load or load range. The computer program was extended to take into consideration the more complex geometry of helical gears using the 'Thin Slice' theory. This allows the prediction of the effects of lead crowning, misalignment, pitch errors and various profile reliefs including relief coined "cross relief", where the relief is applied parallel to the base helix angle. An optimisation routine was included in the computer program that systematically varies the extent of profile relief and amount of lead crowning to minimise transmission error. Since the amount of required profile relief is concomitant with extent of relief and amount of lead crowning (load remaining constant), the program iterates to find the correct amount of profile relief to avoid undesirable comer/tip contact. This new approach has facilitated the generalisation of the effects of profile relief on transmission error for a whole range of typical axial and transverse contact ratios for narrow face width helical gears, previous analyses concentrating on specific designs. Results are also presented for the experimental validation of predicted static transmission error in helical gears. A good correlation between static transmission error, dynamic transmission error and sound pressure level was found as in the spur gear analysis.