This paper reports results of a qualitative study of sixteen students enrolled in a second year organic chemistry course for chemistry and chemical engineering majors. The focus of the study was student use of the arrow-pushing formalism that plays a central role in both the teaching and practice of organic chemistry. The goal of the study was to probe how students made sense of the arrow-pushing formalism by examining their responses to seven organic chemistry problems that required the use of this formalism. This paper discusses common barriers to students' understanding of the arrow-pushing formalism, the concepts and ideas students apply when they use this formalism to solve mechanism problems, and implications of the observation that they used this formalism in a meaningless, mechanical manner. (Contains 4 figures.)