Since Harvard SEAS is relatively new compared to other top graduate engineering programs, its course selection may be limited in some research areas (e.g., non-atmospheric environmental science). As a result, it is quite common for SEAS students to take classes at MIT, which has an extensive course selection.
Your Program Plan should have more Harvard courses than MIT classes. In order to apply to cross-register for a course at MIT, you must submit an online cross registration form, available on the Harvard Course Catalog page. The form will be sent to your instructor at MIT, and he/she will approve/deny your request.
Note that attending classes at MIT will be logistically more difficult than attending classes at Harvard. You will need to physically get to MIT, which may be easiest using Harvard’s M2 shuttle (which makes a few different stops on Harvard’s campus before heading to MIT), public transportation, bicycle, or Uber/Lyft. In addition, class timings at MIT may not align perfectly with class timings at Harvard. For example, MIT’s spring break has historically been offset from Harvard’s spring break by one week.
Also note that you may also cross-register for courses at a few institutions other than MIT (e.g., Tufts and Brown). This route is less common, and the procedure for cross-registering is slightly different; full instructions are available on the Harvard Course Catalog website.
http://handbook.gsas.harvard.edu/cross-registration-courses-offered-othe...
