In this introductory chapter, we covered quite a lot of material. We introduced a mathematical problem, the 4-Queens Problem, and saw in details how the solver manages to find a solution to this problem. We then presented a brief overview of constraint programming and its strengths and weaknesses. The **or-tools** library essentially uses constraint programming. We also gave you
an overview of the beautiful *complexity* theory. And finally, we proposed a school book approach to solve a real problem if you are stuck.