The purpose of this paper is to analyse the relationship between public and private research expenditures since it can provide important information to policy makers to improve the economic performance of country. Data from Eurostat are used. The methodology applies econometric models based on regression analyses. The main results are as follows: Public R&D expenditure is a complement for private R&D expenditure but the latter has to be higher than the former to be a determinant of a country’s productivity growth. These results can be affected by several factors concerning the structure of National Systems of Innovation and Triple helix mechanisms. In addition, this research shows that the composition of public and private investment in research depends on the level of a country’s development.