Greenstein is a leading researcher in the business economics of computing, communications and Internet infrastructure.

His research and writing focus on a variety of topics in this area, including the adoption of client-server systems, the growth of commercial Internet access networks, the industrial economics of platforms, and changes in communications policy.

As a fellow of Berkman Center for Internet & Society at Harvard University he researches slant and bias in Wikipedia.

At Northwestern University he is affiliated with the Center for the Study of Industrial Organization and the Institute for Policy Research.

His wife, Ranna Rozenfeld, is a pediatric critical care medicine physician at Hasbro Children's Hospital in Providence, RI, after many years at Lurie Children's Hospital in Chicago, IL.