Introduction: Despite the fact that the capacity of non-written sources (NWS) understanding is included in the Spanish secondary education curriculum, no research has addressed this competence. The main objective of this study is analysing how teachers consider NWS understanding that should be assessed to help decide which kind of support give to students. Method: First, a model was developed of what NWS understanding implies. Then, 148 primary school teachers in Spain designed tasks to assess the understanding of two NWS. Following this, two judges assessed the proposed tasks according to the initially specified model. We analyzed the fit of teachers' assessment tasks to the proposed model by conducting percentage analyses and a hierarchical loglinear analysis. Results: Teachers' assessments were shown to be unsystematic and were generally composed of tasks of a low level of complexity. Nearly all of them included ambiguous questions or questions which were irrelevant for testing understanding. The proposed tasks corresponded to different profiles, some of which fitted the proposed ideal model. Conclusion: Some changes should be made in teacher education in order to foster students' competency for NWS understanding, especially to detect students' difficulties so that they can be presented with suitable feedback.