Diagnostic competences are essential for teacher actions; however pre-service teachers often do not have the opportunity to train these skills at university. Thus, there is a need to find out the best way to promote diagnostic competences in teacher training. During the last decades, several projects introduced videos as a tool for the training of diagnostic skills, but there is no evidence that pre-service teachers really acquire diagnostic skills better by analysing videos than by analysing tasks. The present study contributes to this growing area of research by exploring which one of these two methods promotes diagnostic skills better. Video analysis and task analysis are compared as training methods in an intervention study with a pretest-posttest design. Fostering preservice teachers' diagnostic skills with focus on students' abilities, problems and misconceptions with graphs of functions, is the specific objective of our study.