Northampton will face either Wasps or Bath in the Amlin Challenge Cup Final after they overcame Harlequins 18-10 at Franklin's Gardens on Friday.
Tries from Tom Collins and Kahn Fotuali'i put the Saints at arm's length of their opposition, despite losing George North to injury in the 20th minute.
Nick Easter did reply with a score for Quins with sixteen minutes remaining of the game - making it 18-10 - but the hosts had enough to hold on.
Saints scored one try in each half and man-of-the-match Fotuali'i was instrumental in both, laying on a cross-field kick to substitute Collins in the 26th minute before scoring a virtuoso try in the 44th.
With Stephen Myler's conversion, Northampton were in an 18-3 lead as the match looked to be heading in only one direction, with England head coach Stuart Lancaster watching on intently.
But Quins, whose full-back Mike Brown did not return to the field for the second stanza, recovered well and when Easter powered over with plenty of time remaining, the comeback was possibly on.
Jim Mallinder's side though held on as, led by strong performances from James Craig and the aforementioned Fotuali'i, they deserved their win and with it a second final this season.
Tries: Collins, Fotuali'i
Pen: Myler 2
Yellow: Elliott (third minute - taking man out in air)
Northampton Saints: 15 James Wilson, 14 Jamie Elliott, 13 George Pisi, 12 Tim Stephenson, 11 George North, 10 Stephen Myler, 9 Kahn Fotuali'i, 8 Sam Dickinson, 7 Phil Dowson (c), 6 Calum Clark, 5 James Craig, 4 Samu Manoa, 3 Salesi Ma'afu, 2 Ross McMillan, 1 Alex Waller.
Replacements: 16 Mike Haywood, 17 Gareth Denman, 18 Tom Mercey, 19 Courtney Lawes, 20 Tom Wood, 21 Ryan Glynn, 22 Luther Burrell, 23 Tom Collins.
Harlequins: 15 Mike Brown, 14 Charlie Walker, 13 Tim Molenaar, 12 Jordan Turner-Hall, 11 Sam Smith, 10 Ben Botica, 9 Danny Care, 8 Nick Easter, 7 Chris Robshaw (c), 6 Luke Wallace, 5 George Robson, 4 Charlie Matthews, 3 Kyle Sinckler, 2 Dave Ward, 1 Joe Marler.
Replacements: 16 Rob Buchanan, 17 Mark Lambert, 18 Paul Doran Jones, 19 Tom Guest, 20 Maurie Fa'asavalu, 21 Karl Dickson, 22 Louis Grimoldby, 23 Ollie Lindsay-Hague.
Referee: George Clancy (Ireland)
Assistant Referees: John Lacey (Ireland), Marius Mitrea (Italy)
TMO: Jim Yuille (Scotland)