A written report on the achievements of scientific research and scientific experiments completed by students or researchers in colleges and universities under the guidance of tutors for obtaining degrees. The degree system originated in medieval Europe. 1 180 The University of Paris awarded the first batch of doctorates in theology. Thesis defense system was initiated by German-speaking countries, and later countries followed suit. All dissertations that pass the defense are generally original research results, which can show the author's professional research ability. Due to the different levels of awarding degrees in different countries' education systems, dissertations are divided into bachelor's dissertations, master's dissertations and doctoral dissertations. Among them, doctoral dissertations have high academic value. In the middle and late 20th century, about 654.38 million doctoral and master's dissertations were produced worldwide every year. Except for a few dissertations published or published after the defense is passed, most dissertations are not published publicly, and only one copy is kept in the library of the degree-conferring university to provide reading and copying services. In order to give full play to the reference function of dissertations, university libraries in some countries make them into microfilms, catalogues and indexes, forming special dissertations databases. There are also a few countries that manage dissertations in a centralized way. For example, British dissertations are kept in the British Library, and only microfilm of the original text is provided to the outside world. Japanese dissertations are also managed by the National Library of Japan. Starting from 1938, American University Microfilm Company edited and published the Monthly Report of International Dissertations, which was divided into Series A (Humanities and Social Sciences) and Series B (Science and Engineering). 1976 added Series C (European Dissertations, Quarterly). The comprehensive index of dissertations 1973 published by the company reports 41861~1972 doctoral dissertations from 400 universities in the United States, Canada and other countries, which can be searched by subject and author name.