The goal of this study was to understand errors that student make when simplifying exponential expressions. College students enrolled in four college mathematics courses were asked to simplify and compare such expressions. Quantitative analysis identified three persistent errors: interpreting negative bases, negative exponents, and parentheses. Qualitative methods were used to examine why they made these errors. Analysis indicated that students frequently misinterpret the negative sign when attached to the base of an exponential expression. We theorize that students' concept image of the negative sign must move beyond rule models to correctly interpret numbers in exponential expressions. [For the complete proceedings, see ED585874.]