This booklet serves as an introduction and companion to the Illustrated Inventory of Medieval Manuscripts. This series is based on four principles. 1. The IIMM aims at completeness, that is exhaustive coverage and so includes fragments as well. 2. Of every manuscript not only a description is given but also a photograph in actual size. 3. The descriptions contain as correct data as possible, and the more the better, but only data which can be collected fairly easily. Simplicity and low cost are required if the IIMM is to be of any use in our lifetime. 4. All descriptions contain the same data, preferably presented in the same manner, and above all collected according to the same rules. Uniformity means a more usable result at greater speed. This booklet also contains the IIMM rules which are to be followed strictly by anyone participating in this project.