This article reports on the investigations with regard to AiF Research Project 14.960 N, 'Utilisation of eddy current technology for groove tracking during the laser welding of zero-gap grooves', which deals with the development of a sensor type and with the set-up of a processing head for laser welding. The zero-gap grooves are no longer detectable with conventional procedures. Several systems for the tracking and welding of zero-gap grooves were implemented within the framework of this research project. The variation of the sensor type and of the construction shape led to the development and optimisation of a suitable sensor system (differential sensor in a T arrangement) which is driven by an eddy current unit. This type of eddy current sensor was selected since, because of its physical working principle and the directional and differentiated field consideration, it is suitable for the exclusive consideration of that secondary field formation in the test specimen which arises due to the welding gap position, to the eddy current distribution and to the magnetisation processes. Furthermore, this construction shape of the sensor permits adaptation to the testing task as well as the separation of the useful and disturbing signal proportions and thus a high detection sensitivity to welding and zero gaps with compensation for disturbing variables. A positioning accuracy of +/- 25 mym in relation to the centre of the gap is achieved with an optimised eddy current T sensor positioning system. In order to stipulate sensor-specific properties, fundamental investigations were conducted in relation to the influences of the variation of the sensor geometry and of the testing frequency with regard to the measuring sensitivity to the welding gap. Furthermore, investigations were conducted into the insensitivity to disturbing variables such as the distance effect, the edge misalignment or the magnetic fields. Various solid-state lasers were combined with fibre lines and the eddy current sensor technology in order to form a processing lens. The laser beam path was modelled according to true-to-scale CAD drawings and was guided to the joint by beam movement devices. At the joint, the laser beam path precisely coincides with the contours to be welded. On the basis of welding tests, it was possible to document the suitability of the system for industrial applications from the project-accompanying committee.