Imagine a country where almost everyone farms, growing just enough food for their own families. Decades later, millions have moved to cities, working in factories and offices, and the whole country is richer. That journey—from farm to factory, from village to city—is called structural transformation. It is one of the most powerful forces in economic development, but it also raises puzzles. Why is it so hard to start? Why does progress sometimes stall? And why do some countries race ahead while others stay stuck? This chapter unpacks ideas that help us understand that journey.