A substantial share of students studying in a face-to-face setting supplement their coursework with online courses, yet we have a limited understanding of online course-taking behavior. This study advances knowledge in this domain by examining an administrative dataset from the Pennsylvania State University. The detailed nature of our data allows for analysis of differences between academic-year and summer-term course-taking and of differences between the incidence and frequency of online course-taking. We find that online course-taking is related to students' characteristics pertaining to time, geography, historical under-representation, and academic preparation. The observed relationships vary between the academic year and summer terms.