PhD students in GSAS students can opt to participate in a number of different secondary fields, as listed on https://www.gsas.harvard.edu/programs_of_study/secondary_fields_in_phd_studies.php.
Typically, a secondary field consists of around 4 courses in a particular discipline, though sometimes the courses can double-count as part of the students regular PhD program. Within SEAS, the most common secondary field is Computational Science and Engineering, which is administered by IACS (http://iacs.seas.harvard.edu ). The CSE Secondary Field emphasizes mathematical modeling and simulation and analysis of large data sets. Interested students should apply by the fall of their third year. The CSE Secondary Field consists of four courses in applied math and computer science and an oral examination. Specific requirements and policies can be found at https://www.seas.harvard.edu/programs/graduate/computational-science-and-engineering/secondary-field-in-cse. Interested students can contact Daniel Weinstock (dweinsto@seas.harvard.edu) for more information.
Another common secondary field among SEAS students is Science, Technology, and Society, which is administered by the Harvard Kennedy School. STS is an interdisciplinary field that prepares students to better understand the interaction between science, technology, and their impacts on society. Students in STS take four courses based on this list: http://sts.hks.harvard.edu/field/courses.html. Interested students can contact sts@hks.harvard.edu for more information.
