It's a staple of news reports from training camp: "On Friday, so-and-so was placed on the PUP list."
As the season nears, talk will turn to avoiding the PUP list—and the six missed games that come with it.
So which PUP should we fear, which one barely merits a raised eyebrow, and most importantly...
What's the deal with all of these dogs?
"PUP" is an acronym for the physically unable to perform list, a landing spot for injured players. As Frank Tadych of NFL.com explained back in 2010, there are essentially two types.
The preseason, or "active," PUP list is for players who start training camp hurt. Players on the active list can be activated at any time and count against the roster limit. Once a player practices, even for five minutes, they cannot be placed on the active PUP list.
Once camp ends, at the second roster cutdown deadline (Aug. 31) teams must decide whether to activate any players on PUP or move them to the "reserve" list. Players moved to the reserve list do not count against the 53-man active roster limit, but those players cannot participate in practices or games for the first six weeks of the season.
After six weeks have passed, teams are given a 21-day window in which they may either activate the player from PUP or place them on injured reserve, ending their season.
Essentially, a player being on the PUP list at this point is no real cause for concern. It's often just a chance to rest dinged-up veterans or players nursing nagging injuries.
However, as the end of August approaches, players who still haven't made it onto the field become real worries. Even then, teams are usually inclined to keep players on the active roster rather than leave them on the PUP list if there's a chance only a game or two will be missed.
Now that we have a working understanding of the PUP list, let's take a look at some prominent players who landed there to kick off camp.
Jay Ratliff, DT, Dallas Cowboys
Ratliff injured his hamstring in pre-camp conditioning drills and immediately headed to PUP. According to Calvin Watkins of ESPN, head coach Jason Garrett said that "we don't anticipate him being on the field until next week and it might be a couple of weeks."
Arian Foster, RB, Houston Texans
Foster strained his calf in offseason workouts, and will open camp on the PUP list as he continues to recover.
However, Foster's injury isn't believed to be serious by any means, and head coach Gary Kubiak told John McClain of The Houston Chronicle that the 26-year-old may only be sidelined "a couple of days."
Ed Reed, S, Houston Texans
Foster isn't the only prominent member of the Texans sitting as camp begins. Veteran safety Ed Reed, who came over from the Baltimore Ravens in free agency, will begin camp on the PUP list as he continues to recover from hip surgery.
If you're looking for an update on his status from Reed, good luck. When asked how he was feeling by McClain, Reed shot back "I'm on PUP. No interviews."
Ahmad Bradshaw, RB, Indianapolis Colts
The Indianapolis Colts signed free agent running back Ahmad Bradshaw in an effort to bolster their running game, but they won't be getting an up-close look at their new ball-carrier just yet.
The Colts are taking it slow with Bradshaw's balky foot. As Dan Hanzus of NFL.com reported, Bradshaw opened camp on the PUP list. Though since the team doesn't begin practicing until Sunday, it's still possible that Bradshaw won't miss much.
Rob Gronkowski, TE, New England Patriots
After having back surgery in June, it's hardly surprising that New England Patriots tight end Rob Gronkowski opened training camp on the PUP list.
The real question with Gronkowski is whether he will recover with sufficient quickness to avoid the regular-season PUP list. The Patriots are being their usual transparent selves in regards to the injury, with head coach Bill Belichick telling Brian McIntyre of Yahoo! Sports that there is "no timetable" for Gronkowski's return.
Marques Colston, WR, New Orleans Saints
As Josh Alper of Pro Football Talk reports, Colston entered camp on the PUP list as the result of a foot injury. However, Alper also reports that the eighth-year veteran is only expected to miss "a few days."
Jason Pierre-Paul, New York Giants
Like Gronkowski, Pierre-Paul recently underwent back surgery that landed him on the PUP list and leaves his Week 1 availability in doubt.
Head coach Tom Coughlin remains hopeful that Pierre-Paul won't miss any regular-season time, telling Ralph Vacchiano of The New York Daily News that "he's done well."
Santonio Holmes, WR, New York Jets
Holmes lost almost the entire 2012 season to a Lisfranc injury that now jeopardizes his 2013 campaign as well.
That's the word from Manish Mehta of The New York Daily News, who writes that Holmes was noncommittal when asked if he would play this year, stating that "I can't answer that question right now."
Heath Miller, TE, Pittsburgh Steelers
The veteran tight end's career year in 2012 was cut short by an ACL tear, and the timing of that injury clouds Miller's availability for early in the 2013 season.
Miller heads into camp firmly planted on the PUP list, and Ralph Paulk of The Pittsburgh Tribune-Review points out that at this point there's still a decent chance that Miller may open the regular season on the PUP list as well.
Michael Crabtree, WR, San Francisco 49ers
If there's a player on this list who's more or less a lock to open the season on the PUP list, it's Crabtree, who tore his Achilles' tendon in offseason workouts.
However, head coach Jim Harbaugh recently told Matt Maiocco of CSN Bay Area that Crabtree's recovery has "looked great" so far, and the 49ers still hold out hope that Crabtree will play at some point in 2013.
Percy Harvin, WR, Seattle Seahawks
Without question the biggest PUP list bombshell was the inclusion of Harvin, who the Seahawks acquired in a blockbuster offseason trade.
According to Hanzus, Seattle head coach Pete Carroll revealed Thursday that Harvin injured his hip in workouts a week ago. Surgery that could potentially end the fifth-year pro's season has not been ruled out.
ESPN's Adam Schefter reports that Harvin will meet with Dr. Bryan Kelly in New York on Tuesday to get a second opinion as to whether surgery is the best course of action.