The use of linkages to compare productive structures and discover key sectors is an active focus of research in input-output field. In this paper, an extension of the called multilevel indicators (García et al., 2008) to the key sector determination is proposed. The multilevel indicators not only describe industries with a strong influence on the expansion of other sectors in an economy but the immediacy and the transmission capacity of their impacts. The proposal opens a new inquiry about how the relational structure affects the contagion diffusion and the robustness of the economic system. The empirical key sector analysis will focus on the Greek economy. The study deals with the structural change of Greek economy in the last decade (2000-2010). Conclusions about the relevance of some activities for the development of Greek economy are offered