Greg has a long-standing interest in the culture of empire in the ancient world. He has worked on the formation of provincial cultures, often using archaeological material, and also on the cosmopolitan cultures of the metropolis. Much of his work considers the Roman world in a global perspective.

He has written on literacy, on knowledge cultures and libraries, on ethnography, on the Roman economy, on gendered Roman history and on the emergence of religions. His latest book The Life and Death of Ancient Cities. A Natural History, reflects a growing interest in the history of the very long term. Currently he is working on a book on migration and mobility, and also on urban resilience as one aspect of the environmental history of antiquity.