[2] It was presented by Caroline Flack and Olly Murs, who had both previously co-presented spin-off show The Xtra Factor on ITV2 and replaced Dermot O'Leary, who left after eight series.

[4] Rochelle Humes and Melvin Odoom presented The Xtra Factor, replacing Sarah-Jane Crawford.

In March 2015, Simon Cowell was confirmed to return as a judge for the twelfth series, his ninth on the show.

[5] On 14 May 2015, Walsh confirmed his exit from the show after 11 series on the judging panel, stating, "The truth is I've done it for 11 years; I never thought I would even be on TV for four or five.

On 16 April, ITV confirmed that former contestant Olly Murs and Caroline Flack would take over presenting the show, having worked together as hosts before on The Xtra Factor.

[4] On 18 June 2015, it was confirmed that The Saturdays singer Rochelle Humes and Kiss FM DJ Melvin Odoom would be the new hosts of The Xtra Factor.

Auditions ran between 30 March and 23 May 2015, and visited Aberdeen, Skegness, Bradford, Huddersfield, Dundee, Sheffield, Peterborough, Northampton, Stirling, Cambridge, Middlesbrough, Nottingham, Scarborough, Norwich, Leicester, Ipswich, Coventry, York, Kingston upon Hull, Oxford, Chelmsford, Berkshire, Hertfordshire, Southend-on-Sea, Belfast, Bangor, Southampton, Isle of Man, Swansea, Blackpool, Bournemouth, Carlisle, Brighton, Truro, Isle of Wight, Wigan, Plymouth, Broadstairs, Margate, Stoke-on-Trent, Exeter, Essex and Yeovil.

[16] The first day of London auditions were scheduled to be recorded on 14 July, but were cancelled due to the aforementioned bereavement.

[19] Filming was delayed on 20 July due to Fernandez-Versini suffering a burns injury to her foot after stepping on some hair tongs.

[26] Grimshaw was absent from the second day of the challenge due to his commitments with BBC Radio 1, although partially assisted Cowell, Fernandez-Versini and Ora with their decisions via Skype video link.

[29] It was reported that the standard of auditions were so high that acts who got a "yes" from all four judges were not necessarily guaranteed a place at bootcamp.

[30] For the first time in X Factor history, viewers voted via hashtags on Twitter to determine which of the judges was allocated each of the four categories (for example, #SimonBoys or #RitaGroups).

[31] Ora was in charge of the Girls, Cowell was assigned the Over 27s, Fernandez-Versini had the Groups and Grimshaw looked after the Boys.

At the start of the challenge, the judges discovered which categories they would mentor: Ora was given the Girls, Cowell was given Over 27s, Fernandez-Versini was given the Groups and Grimshaw was given the Boys.

The new names were revealed as 4th Impact (4th Power), Alien Uncovered (Alien), BEKLN Mile (BEKLN), Melody Stone (Silver Tone), New Kings Order (The First Kings) and Reggie 'n' Bollie (Menn On Point)[35] The X Factor resumed its usual Saturday and Sunday night slots for judges' houses, taking place over one weekend (24–25 October).

Judges' houses was planned to be broadcast fully live, and recorded back-to-back,[36] but on 15 September, it was announced that this would not happen as it would cause logistical problems.

[38][39] Girls: Boys: Groups: Over 27s: After being eliminated, Monica Michael was reinstated as a wildcard before the live shows.

ITV were contractually obliged to show all of Home Nation's Rugby World Cup games in October, some of which take place during The X Factor‍'s Saturday night slot.

The Xtra Factor moved its Saturday episode to Thursdays, being broadcast before the live shows, starting on 29 October.

[citation needed] Series 11 runner up Fleur East and CeeLo Green performed on the second live results show on 8 November,[citation needed] whilst Jess Glynne[46] and series 7 contestants One Direction[47] performed during the third live results show on 15 November.

[48] The quarter-final results show on 29 November featured performances from Carrie Underwood, and Sigma and Ora.

[50] The live final on 12 December featured duets from Leona Lewis and Ben Haenow,[51] and Little Mix and Fleur East.

As a mark of respect following the November 2015 Paris attacks it was announced that Lauren Murray and Monica Michael would change their songs.

Voting opened at the end of the fourth live results show, and closed at midnight that night.

1 Includes advert breaks 2 The ITV+1 figure for this episode is unavailable as it was outside the top 10 programmes of the week on BARB.

A spokesperson for the show said this was "kinder and fairer than an immediate 'cull' of singers on arrival at bootcamp, which has happened in previous series.

"[98] During the six-seat challenge, after performing for his mentor Grimshaw, Noise complained that his audition was only broadcast for 47 seconds while other contestants were featured for up to 10 minutes.

We've seen this year's line-up undertake a big shake-up and Olly and Caroline were considered to be a safe bet."

"[55] Isabel Mohan from The Daily Telegraph said, "We were always dreading Flack and Murs taking over from Dermot O'Leary—we knew they wouldn't be able to hack it live on primetime telly.

"[55] O'Leary supported Murs on Twitter, saying it "Happens to us all" and "live TV is a tricky old beast at times.