The author of the Handbuch zur lateinischen Sprache des Mittelalters contemplates in retrospect the development of his work, that engaged him for some twenty years, and gives an account of the great usefulness of the existing medieval Latin dictionaries. Details about the planning and the gradual accomplishment of the different parts are disclosed together with the heuristic methods employed and the guiding deliberations in the composition of the work. This ambitious project could only find a successful end, because the author restricted himself in some respects from the onset. Although the Handbuch pursues different objectives than the medieval Latin dictionaries, there are, due to its very nature, close contacts. The respective qualitative and quantitative use that was made of them, including those still in progress, as sources for the Handbuch is evaluated in this article. The author has close ties in particular to the Mittellateinisches Wörterbuch based in Munich, where he had been introduced to the lexicographical practice and whose card files rendered him excellent services for his Handbuch.