Introduction 1 Lecture 1. Gromov-Witten theory 2 Lecture 2. Orbifolds / Stacks 16 Lecture 3. Twisted stable maps 20 Lecture 4. Gromov-Witten classes 28 Lecture 5. WDVV, grading and computations 35 Appendix A. The legend of String Cohomology: two letters of Maxim Kontsevich to Lev Borisov 42 References 45