This book grew out of my dissatisfaction with books on distributed systems that included only pseudo-code for distributed algorithms. There were two problems with pseudo-code. First, pseudo-code had many assumptions hidden in them making them more succinct but only at the expense of precision. Second, translating pseudo-code into actual code requires effort and time. Result --- students never actually saw executions of these algorithms. Seeing the code run lends an extra level of confidence into one's understanding of the algorithms. In this book, I have used actual Java source code for explaining distributed algorithms. Students can run, modify and experiment with them.