This study investigated high school students’ misconceptions on basic mathematics skills, and how these misconceptions influenced their higher-level skills over the course of an Algebra unit. This research was conducted in an urban classroom in Illinois, within four Algebra II classes and one Geometry class. Students took five mastery quizzes to assess their basic mathematics skills over the course of a semester. Mastery quizzes and teacher journals were content analyzed (Neuendorf, 2002) to determine common misconceptions among students in the areas of fractions, decimals, percentages, integers, and scientific notation. Findings illustrate common misconceptions of basic mathematics skills among high school mathematics students and how these affect their learning of advance materials.