This paper presents a unifying scenario classification model and a selection of scenarios, developed within the EU FP7 QUASAR project to study secondary spectrum usage. The classification model categorizes scenarios from technical, regulatory, and economic perspective. It enables the derivation of the most promising scenarios of secondary spectrum access: Cellular and WiFi-like usage of TV white spaces, wireless backhauling using secondary spectrum access, license exempt use of the radar bands, use of the aeronautical bands for mobile broadband and cognitive machine-to-machine communication. These scenarios are presented in more detail along with motivations of why they are interesting to study further. The scenarios will provide the basis for the future work within the QUASAR project, which has the overall objective to assess the amount of useful spectrum opportunities for secondary spectrum access.