This research considers the first year of a longitudinal study in which we are investigating the development of children's algebraic thinking as they take part an early algebra intervention in kindergarten through grade 2. Specifically, this study investigates kindergarteners' understanding of unknown quantities and their representation of these quantities before and after an early algebra intervention. Findings show that as early as kindergarten, many students can develop understandings of unknown values, as well as use variable notation to describe unknowns. Students in the study who did not yet use variable notation were largely able to coordinate assigned quantities accurately, which we believe provides a solid foundation for writing variable expressions to describe unknown quantities.