This article is concerned with the role of experientia in medieval law. This could take a number of forms, three of which are treated by way of example. In the first part of the essay, the author discusses how experience first came to serve as a source for the creation and legitimisation of new law since Late Antiquity. Henceforth, it became an important principle within Canon Law that served not only to create legal regulations supplementary to traditional law but also (...) to overwrite and replace it. The second part of the essay is concerned with the role of expert reports (consilia) by men learned in the ius commune which were requested to help rule cases under local law. Being dispersed in collections after their use in court, consilia made a significant impact on common and local law. In the final section the author discusses the use of expert witnesses (from areas as diverse as medicine, agriculture, the arts and crafts etc.). Expert opinions were recognised by court as high-value evidence. (shrink)