Evaluations of procedures in medical image processing are notoriously difficult and often unconvincing. From a detailed bibliographic study, we analyzed the way evaluation studies are conducted and extracted a number of entities common to any evaluation protocol. From this analysis, we propose here a generic evaluation model (GEM). The GEM includes the notion of hierarchical evaluation, identifies the components which have always to be defined when designing an evaluation protocol and shows the relationships that exist between these components. By suggesting rules applying to the different components of the GEM, we also show how this model can be used as a first step towards guidelines for evaluation.