The thesis of this book is that cities are the primary drivers of economic development. Her main argument is that explosive economic growth derives from urban import replacement. Import replacement occurs when a city begins to locally produce goods that it formerly imported, e.g., Tokyo bicycle factories replacing Tokyo bicycle importers in the 1800s. Jacobs claims that import replacement builds up local infrastructure, skills, and production. Jacobs also claims that the increased production is subsequently exported to other cities, giving those other cities a new opportunity to engage in import replacement, thus producing a positive cycle of growth.
In the foremost chapter of the book, Jacobs argues that cities preceded agriculture. She argues that in cities trade in wild animals and grains allowed for the initial division of labor necessary for the discovery of husbandry and agriculture; these discoveries then moved out of the city due to land competition.