I was invited to lecture at the 1998 Marktoberdorf summer school. One reason I accepted was that I was in the process of writing a book on concurrency, and I could use the early chapters of the book as my lecture notes. However, I decided to put aside (perhaps forever) that book and instead write a book on TLA+. I was able to recycle much material from my original notes for the purpose. For the official volume of published notes for the course, I decided to provide this, which is a preliminary draft of the first several chapters of .