Currently, miles will not expire as long as you participate in one of the following activities at least once every two years; mileage expires midnight Eastern time (-5 GMT), 24 months from the date of the last activity:
Earn miles for travel on a qualifying Delta, Delta Shuttle®, SkyTeam, or other SkyMiles airline partner flights.
Earn or redeem miles with one of the SkyMiles program partners including hotels, car rentals, Delta SkyMiles Credit Card from American Express, international credit card partners, SkyMiles Dining & Hotels by Rewards NetworkSM, MilePoint.com, mortgage lenders, CAP magazine subscriptions, real estate, or telecommunications partners.
Purchase miles through SkyMiles Buy Miles.
Receive miles through SkyMiles Gift Miles.
Transfer miles (or receive transferred miles) through SkyMiles Transfer Miles.
Redeem miles on Delta, Delta Shuttle, a SkyTeam partner, or another SkyMiles airline partner.