On September 5, 2023, New York City implemented Local Law 18, or the Short-Term Rental Registration Law. This thesis examines how the opportunity cost of hosting short-term rentals on online marketplaces changed after the enforcement of this law. I develop a structural supply model and estimate daily market-level cost distribution parameters across different categories of Airbnb rentals. For short-term rentals of less than 30 days, the cost distribution means increase sharply at enforcement. Thereafter, costs stabilize at a level above the pre-enforcement average for partial-property listings, but exhibit a gradual decline over time for entire-property listings. For rentals of over 30 days, the seasonal variation in costs remains unchanged. These patterns are consistent with expectations based on the regulation’s specific provisions. They characterize the supply-side response to regulation, which can inform future work in modeling short-term rental markets and policy.