The beginning of the second Italian Research Evaluation Exercise (known as VQR 2011-2014) is an opportunity to reflect on evaluation criteria used to assess research products, taking into account the previous experience as well: the VQR 2004-2010. The central concept of VQR is the quality of research, as its definition is essential for evaluation. The definitions of the criteria for the VQR 2011-2014 (originality, methodological rigor and certified or potential impact), and for the VQR 2004-2010 (relevance, originality/innovation and internationalization) are analyzed from a methodological point of view. This reflection does not touch the objectives of the evaluation nor does it discuss the choice of the criteria; instead it considers the definitions of the evaluation criteria following the scheme proposed by Sartori for concept analysis.