It's an excellent question and one of the many that have surfaced since Swift started touring the world in support of her quintuple-platinum "1989" album. The pop mega-star has always made a habit of inviting an eclectic group of celebrity guests on stage at her concerts, but this year, she has raised the bar with a barrage of singers, supermodels, athletes, A-list actresses and famed '90s sitcom stars. Some perform, some just stand on stage and wave. Not only is it a treat for the audience, but it serves the guests very well. After all, history shows that being associated with Swift can greatly impact your celebrity status.
It's a lot to process, so here's a complete guide to the guests on Swift's tour — and why these stars were the chosen ones.
- Las Vegas, May 15
Musical guest: Ed Sheeran
Song: "Tenerife Sea"
Why was Sheeran there? Swift and Sheeran go way back, from that time they wrote her single "Everything Has Changed" on a trampoline. Plus, he was an opener on her "Red" tour.
- Detroit, May 30
Detroit
Musical guest: Dan Reynolds of Imagine Dragons
Celebrity guests: Martha Hunt, Gigi Hadid
Song: "Radioactive"
Why was Reynolds there? Imagine Dragons is one of her favorite bands, as she told the crowd.
Why were Hadid and Hunt there? Both model squad members and co-stars in the "Bad Blood" video, Hadid is also dating Swift's ex-boyfriend, Joe Jonas.
- Pittsburgh, June 6
Musical guest: Little Big Town
Song: "Pontoon"
Why were they there? Swift is still a Nashville gal at heart.
- Philadelphia, June 12
Musical guest: Echosmith
Celebrity guests: Cara Delevingne, Mariska Hargitay
Song: "Cool Kids"
Why was Echosmith there? Swift and lead vocalist Sydney Sierota became pals at the 2014 VMAs.
Why were Delevigne and Hargitay there? Delevingne is a longtime member of the #squad; Swift is a huge "Law & Order: SVU" fan, and one of her cats is named after Hargitay's character, Olivia Benson.
- Philadelphia, June 13
Musical guest: Rachel Platten
Celebrity guest: Mariska Hargitay
Song: "Fight Song"
Why was Platten there? Swift bonded with Platten backstage a few nights earlier in Pittsburgh singing this inspirational tune.
Why was Hargitay there? See above.
-London, June 27
Celebrity guests: Cara Delevingne, Gigi Hadad, Martha Hunt, Kendell Jenner, Karlie Kloss, Serena Williams
Why were they there? All squad members, except for Serena Williams, who was in town for Wimbledon.
- East Rutherford, New Jersey, July 10
Musical guest: The Weekend
Celebrity guests: Lily Aldridge, Lena Dunham, Gigi Hadid, Heidi Klum, Hailee Steinfeld, U.S. women's soccer team
Song: "Can't Feel My Face"
Why was The Weeknd there? Well, they do share a producer in Max Martin.
Why were all those other people there? It's a good number of "Bad Blood" video stars and friends, plus Klum, who likely felt right at home among fellow supermodels Aldridge and Hadid. Oh, and the women's soccer team had just won the World Cup.
- East Rutherford, New Jersey, July 11
Musical guest: Nick Jonas
Celebrity guests: Uzo Aduba, Lily Aldridge, Gigi Hadid, Martha Hunt, Karlie Kloss, Behati Prinsloo, Candice Swanepoel
Song: "Jealous"
Why was Jonas there? Swift and the JoBros have known each other for years; he also dated her BFF Selena Gomez.
Why was everyone else there? Some repeats from the night before, with the addition of some more supermodel friends and Uzo Aduba from "Orange is the New Black." (Maybe Swift is a Netflix binge-watcher?)
- Washington, D.C., July 13
Musical guest: Lorde
Song: "Royals"
Why was Lorde there? Lorde & Taylor: A well-documented Insta-friendship.
- Washington, D.C., July 14
Musical guest: Jason Derulo
Song: "Want to Want Me"
Why was Derulo there? Swift told the crowd it's one of her favorite summer songs.
- Chicago, July 18
Musical guests: Andy Grammer, Serayah
Song: "Honey, I'm Good"
Why was Grammer there? This is his second appearance on a Swift tour — he showed up a couple years ago to play "Keep Your Head Up," Swift's favorite song when she's having a bad day.
Why was Serayah there? A fairly new edition to the squad, the 20-year-old "Empire" star helped Swift out on vocals during "Style."
- Chicago, July 19
Musical guest: Sam Hunt
Celebrity guests: Lily Donaldson, Andreja Pejic
Song: "Take Your Time"
Why was Hunt there? Swift likes to keep tabs on the Nashville scene. Before their flirty duet, she called him "the coolest new thing that country music has going on."
Why were Donaldson and Pejic there? Because Swift really, really likes having supermodel friends?
- Foxborough, Massachusetts, July 24
Musical guest: Walk the Moon
Song: "Shut Up and Dance"
Why was Walk the Moon there? Released last year, it's still a contender for song of the summer.
- Foxborough, Massachusetts, July 25
Musical guest: MKTO
Song: "Classic"
Why was MKTO there? Because it's the catchiest song ever and earworm expert Taylor Swift knows it.
- Vancouver, Aug. 1
Musical guest: Nico & Vinz
Song: "Am I Wrong"
Why were Nico & Vinz there? Swift is OBSESSED with them.
- Seattle, Aug. 8
Musical guest: Fetty Wap
Celebrity guests: Ciara, Russell Wilson
Song: "Trap Queen"
Why was Fetty Wap there? Another song of the summer contender.
Why were Wilson and Ciara there? Swift likes to bring out hometown stars (Wilson is a favorite of the Seahawks) and he and Ciara are one of this year's most-talked about couples.
- Santa Clara, California, Aug. 14
Musical guest: Fifth Harmony
Song: "Worth It"
Why was Fifth Harmony there? Swift has been good friends with group member Camila Cabello for awhile, which she proved by borrowing one of Fifth Harmony's outfits for the performance.
- Santa Clara, California, Aug. 15
Musical guest: Little Mix
Celebrity guests: Joan Baez, Julia Roberts
Song: "Black Magic"
Why was Little Mix there? They were already nearby for the Teen Choice Awards in Los Angeles. Oh, and group member Perrie Edwards used to be engaged to Zayn Malik, the ex-One Direction star, and Swift also used to date Harry Styles.
Why were Roberts and Baez there? Your guess is as good as ours — though Swift did call the women two of her heroes.
- Los Angeles, Aug. 21
Musical guest: Ryan Tedder
Celebrity guest: Kobe Bryant
Song: "Counting Stars"
Why was Tedder there? He co-wrote two tracks on "1989" ("Welcome to New York" and "I Know Places") so it was only fair.
Why was Bryant there? In a twist, the Lakers star was there to surprise Swift: He presented a new Staples Center banner that congratulated her on being the artist with the most sold-out concerts ever at the venue.
- Los Angeles, Aug. 22
Musical guests: Mary J. Blige, Uzo Aduba
Celebrity guests: Matt LeBlanc, Sean O'Pry, Chris Rock
Blige's songs: "Doubt," "Family Affair"
Why was she there? Swift says she was very inspired when she heard Blige sing "Doubt" at a pre-Grammy party last year, and also wants no more drama in her life.
Aduba's song: "White Horse"
Why was she there? Aduba (a guest earlier in the summer in East Rutherford) told Vulture she got a message from Swift inviting her to sing on stage. Who would turn that down?
Why were LeBlanc, O'Pry and Rock there? Swift is a big fan of "Friends." We know Rock is a big fan of Swift. And Sean O'Pry was the star of Swift's "Blank Space" video.
- Los Angeles, Aug. 24
Musical guest: Alanis Morissette, Natalie Maines
Celebrity guest: Ellen DeGeneres
Maines' song: "Goodbye Earl"
Why was she there? Swift used to perform Dixie Chicks songs at all her school talent shows, so may as well bring it to a bigger stage.
Morissette's song: "You Oughta Know"
Why was she there? It's one of the most famous songs about an ex-boyfriend, a theme Swift knows very well.
Why was DeGeneres there? The two have been buddies for years, with Swift a frequent guest on her talk show — even though Ellen often tries to freak her out.
- Los Angeles, Aug. 25
Musical guests: Beck, St. Vincent, John Legend
Beck's song: "Dreams," with St. Vincent on guitar
Why were they there? St. Vincent is dating Swift's squad member Cara Delevigne; Beck has a new single.
Legend's song: "All of Me"
Why was he there? According to Swift, she heard Legend and his wife, Chrissy Teigen, were on their way to the show — so she called and asked about an impromptu performance.
- Los Angeles, Aug. 26
Musical guests: Justin Timberlake, Selena Gomez
Celebrity guest: Lisa Kudrow
Timberlake's song: "Mirrors"
Why was he there? The former 'N Sync frontman was one of Swift's first celebrity crushes before she became a superstar — now he joins her on stage and shows up at her 25th birthday party.
Gomez's song: "Good For You"
Why was she there? The two have been best friends for a long time, even though Swift is rightly repulsed by Gomez's on-again off-again romance with Justin Bieber.
Why was Kudrow there? To sing "Smelly Cat" from "Friends" — that's right, Swift's powerful enough to get an Emmy-nominated actress to reprise a decades-old sitcom role.
- San Diego, Aug. 29
Musical guets: Avril Lavigne, OMI
OMI's Song: "Cheerleader"
Why was he there? It's another song of the summer contender.
Lavigne's song: "Complicated"
Why was she there? To quell the notion that the two are in a feud. People tried to stoke the fire when Swift "liked" a Tumblr post that implied Lavigne wasn't friendly to her fans — but Swift is way too savvy for that story to blow up.
