We describe a focus control system for Nd:YAG laser welding based on a non-intrusive optical sensor incorporated into the fibre delivery system to detect light generated by the process. This broadband light is separated into two wavelength bands. Simple electronic processing gives an error signal proportional to focal error as a result of chromatic aberrations in the optical delivery system. Focus control is demonstrated for bead-on-plate welds in different materials of varying thicknesses and at various welding speeds. The system was demonstrated with both pulsed and CW laser welding.