On the market there is confusion about a large number of potential counterfeits of old masters. This raises questions about the way in which the authenticity of paintings of old masters is determined and how the method(s) can be improved. In this research project, digital tools are developed that facilitate tracking of similarities and differences in paintings, both in the visible area and in deeper layers. To do this, insights from art history are combined with chemical research techniques and recent developments in computer science. The tools are developed on the basis of three case studies. In this way, new insights are developed in the paining technique and studio practice of Frans Hals (1582/83-1666), and controversial pieces are compared in a new, deeper way to uncontested originals of the same type. The tools are tested in collaboration with an international group of experts.