While plagiarism by college students is a serious problem that must be addressed, students generally overestimate the frequency of plagiarism at their schools and blame students they do not know for the majority of incidents. This study looked at students' estimations of the frequency of plagiarism at a large urban college and explored how that varied over the full range of types of plagiarism, from using another author's ideas to submitting an entire document copied verbatim from another author's work. Analysis of student responses to survey items revealed they believe other students are far more likely than them to commit each type of plagiarism and they recognize that some types of plagiarism are more serious than others. The opportunity to reduce incidents of plagiarism by providing students with accurate information about plagiarism at their schools is discussed in the context of social norms theory.