In the scholarship of Western music various non-written processes of learning, adopting, perceiving, making or teaching have generally been more or less ignored. This work is an attempt to introduce some interdisciplinary methodological possibilities, obtained from contemporary ethnomusicology and cognitive musicology, to the research of early music. The reference material and methodology deal to a great extent with the dialectic between the oral and the written in music; different levels and kinds of variation and variable factors within various structural hierarchies of music; adoption and transmission processes of music; processes of change; culture studies. In this study the existence and importance of oral adoption and transmission within medieval Welsh harp music tradition has been one of the main hypotheses. That tradition was based upon highly educated professional musicianship, culminated in the degree of pencerdd i.e. 'the chief of music or craft'. Medieval Welsh harp music was essentially an oral music culture, of which very little evidence in musical notation has survived. The main piece of musical evidence is the Robert ap Huw manuscript from c. 1613. A piece of music in that manuscript is not necessarily the original creation of its attributed author, but a version comprising numerous rhythmic, melodic or other changes over a period of time. In connection with medieval Welsh harp music we may speak of culturally correct and hierarchically structured referential frameworks for creating, learning and making music. The hierarchical relationship of various elements of musical expression helped the musician to employ his toolkit for composing, performing, ornamenting or improvising. Such hierarchical systems, together with their adoption through systems and rules of education, was the essential foundation which enabled musicians to keep the music alive and in memory for centuries, as suggested in my conclusion. Some reconstruction attempts, based on the research results, together with contemporary musical experiments on medieval Welsh harp music are presented in the joint CD of this study.