Herskovits sets out to make a working concept of a term anthropologists had been using loosely. He tests the competing definitions and shows that separating acculturation from assimilation by whether borrowing runs one way or both, or by whether a people chose freely, collapses under actual cases: the Pawnee, moved to a reservation and taught to farm, cannot be classified either way.

He settles on the formulation drafted for the Social Science Research Council, under which acculturation covers the changes that follow when groups of different cultures enter continuous first-hand contact, and defends its deliberate silence about which culture dominates and whether the exchange is reciprocal. It must be kept distinct from culture change at large and from diffusion.

The book then turns on the existing literature, weighing monographs on Native American and southern African peoples for how far each catches contact as a process rather than reconstructing it in retrospect.