Some services and activities in Glacier National Park either require reservations or reservations are recommended.

Lodging
Lodges fill up quickly and reservations are very strongly encouraged. Addresses and phone numbers are found on the lodging page.
Lodging Page

Campgrounds
Most campground in Glacier are first-come first-served, but two (Fish Creek and St. Mary) may be reserved in advance. For information on applying for a campground reservation please contact:
National Recreation Reservation Service

Backcountry Permits
Starting April 15 permit applications will begin to be processed. It is recommended that if you are planning a backcountry camping trip in Glacier, you apply for a permit. Nearly half of all the backcountry campgrounds are held for walk-ins, but those with reservations have a better chance of obtaining thier desired campsites.
Backcountry Camping Page

Other Services
Bus tours, guided hiking tours, horseback rides, and boat trips are often available without reservations, but if your trip plans are not flexible you might want to contact the companies providing those services directly and make reservations prior to coming to the park.
Outdoor Activities Page