Innovations of agricultural suppliers, producers and retailers are directly or indirectly shaping sustainability within the agro food web. If sustainable innovations targeted at the key challenges agriculture is facing worldwide, such as food security, climate change, sustainable use of natural resources etc. should be promoted, knowledge about current innovation processes is needed to reveal mechanisms that allow for promoting sustainable agricultural innovations. In this paper we present the development of an analytical framework to study agricultural innovation systems. We divide the agricultural sector into four levels and expand the innovation system approach (Malerba 2002 and 2004, Koschatzky 2009) to study innovation processes. On the example of the role of farmers and extension services in agricultural innovation processes we demonstrate the adequateness of the approach and give detailed insight into the later stages of the innovation process, where barriers occur most in the German agricultural innovation system.