Conceptual modeling, deciding what to include in the model, is a very important task in the modeling process. However, it has so far received relatively little attention in the literature and there is a lack of empirical data. This paper describes three empirical studies on conceptual modeling and the modeling process. In the first one, data on the time spent on different topics during real simulation projects by an expert and by groups of novice modelers was collected and analyzed. The second study was a questionnaire survey to obtain data on conceptual models and modeling processes for projects carried out by experienced modelers. The third study was an experiment to investigate the effect of model size on the ease of understanding of the model.