Permits are required year-round for all overnight stays in Yellowstone's backcountry. A backcountry permit allows the permit holder and group members to camp in a designated location. Permits are not required for day hiking.

https://www.nps.gov/maps/full.html?mapId=f926f448-9fe4-4600-b4c0-3f060737c87f

Peak Season (May 15 - November 5) - during the peak season backcountry permits are available online in advanced or in-person during the Walk-up period. Advanced reservations are available at Recreation.gov during the Early Access Lottery and General On-Sale period. Walk-up permits are available in-person at a backcountry office up to 2 days prior to start of trip.
Non Peak Season (November 6- May 14) - backcountry permits for the non peak season are available online at www.recreation.gov starting October 6, or in-person no more than two days prior to your trip at the South Entrance (Mid December-Mid March). Permits may be printed or downloaded at home within 5-days of the trip start date after receiving a backcountry orientation from the backcountry office staff. Visit the Winter Backcountry Camping page for more information regarding obtaining a backcountry permit during the winter season.

Heart Lake Trail - Yellowstone National Park — Yellowstone National Park

Heart Lake in Yellowstone's backcountry can be accessed from several access points within the park and can be coupled with another night (or multiple nights) in the backcountry to really get a sense of the grandeur of America's first National Park.

The Heart Lake backcountry sites can be reserved ahead of time through a permitting system that processes mailed in reservations only. There is a possibility that you can walk into a ranger station the day of and acquire a permit but there are no guarantees. I would recommend utilizing the advanced permitting service which requires a slightly higher fee.

The campsites at Heart Lake are just a short distance away from the lake itself, many of them only a few steps. There are no bear boxes in this area so you absolutely need to carry a bear can with you. Grizzly Bears are prominent here and signs of their frequenting can be found all around. The site we stayed in had a nice flat area with a  few log benches and we were able to find a level sight far enough away from the cooking area to pitch our two person tarp. The lake is stunning and worth taking a short dip in (it's cold) after a long day of hiking. The sites do have primitive composting toilets. They are not enclosed like many backcountry pit toilets, just an elevated toilet seat so be sure to bring toilet paper.

Sites are well spaced from one another and almost impossible to see others in neighboring sites. There was also a small flowing stream feeding into the lake near our site that was easy to get water from for filtering and cooking. 

If you are looking for a way to get out of the crowds, take a hike into Heart Lake for the evening!