I was originally induced to think about barriers to entry by some re­ search done for Charles River Associates, Inc., Boston, on behalf of their client IBM Corporation, Armonk, N.Y. After the end of this re­ search project I continued to work on entry barriers and related is­ sues of market structure. The results of this effort are reported here. What I present is not a book in the traditional sense of the word. Due to other research and administrative obligations I could not, without substantial delay, present a more finished product. The rapid progress in the field of industrial organisation theory made me fear that my results could become obsolete if I further waited with their publica­ tion. I hope that the early presentation of a progress report rather than a much later publication of a finished product is not only in my interest but also in the interest of economics.