It is argued that the literature on scientific research and development outsourcing focuses mostly on client firms, that is, the demand side, while little is known of the characteristics of the supplying sector. The present paper tackles this gap by elaborating an exploratory analysis of this business service sector with a view to analyze the main patterns of specialization and their evolution over time. Using data on job content and skill requirements in the United States, we elaborate a knowledge taxonomy of the sector and explore how different forms of knowledge coexist and coevolve.