The book evolved from the experience of the authors gained from teaching the Introduction to Computer Science course, offered at the Math and Statistics Institute of the University of São Paulo (home of the Computer Science Department). Its main characteristics are the informal style and the use of object-oriented concepts right from the first class on. This book has already been adopted as the textbook for the introductory computing course for students enrolled in Computer Science, Mathematics, Information Systems, and Physics undergraduate courses for the past 6 years. The book is made available under a Creative Commons license and therefore may be freely downloaded. The course is based on DrJava, a free software environment to support teaching programming.