We encounter ratios on a daily basis. They also play an important role as a basic construct of thinking in many areas of school mathematics. For example, a fraction can be interpreted as the ratio of a part to the respective whole. Many children appear to have difficulties with fractions and although the concept of ratios is crucial for this subject area, there has been hardly any scientific research on how the understanding of ratios is developed. In this article, we will highlight, using the "marbles problems", how children between 3rd and 6th grade handle ratios. [For the complete proceedings, see ED597799.]