The world has urbanized, but it has not done so exclusively or even mainly in large cities. Almost 2 billion people, 27% of the world’s total population or half of the world’s urban population, reside in towns and small and medium cities of up to half a million inhabitants. An additional 3.4 billion people are classified as living in rural areas, or 46% of our planet’s inhabitants. The majority of the world’s poor, perhaps as many as 70%, live in these towns and small and medium cities and the rural areas more proximate to them, and poverty rates are also higher in small and medium cities than in large urban agglomerations. This desk review is about the relationships involving 5.5 billion persons, three quarters of all of us on Earth, that live in the increasingly diffuse and porous interface of rural and urban societies. The study was commissioned by The Ford Foundation, and the terms of reference directed the authors to focus on four entry points to review rural-urban linkages: the changing nature and borders of rural and urban, food systems, labor markets, and domestic migration. The report is based on a thorough review of the literature on these four specific subjects (384 publications are referenced), seven commissioned papers and a limited number of interviews of experts from around the world.