Who is/isn't the mother?

Robin. Well I think we can for sure say that Robin is not the mother. In Season 3 episode 12 (No Tomorrow), Future Ted says

I found out years later that your mom was at that party. I just didn't meet her.

This definately rules out Robin as she was at Marshall and Lilly's House warming that night. UNDEBATABLE! It is not Robin

Any woman Ted knows or dated before Season 2 episode 15 (Lucky Penny). At the end of the episode, Future Ted says

My destiny was to stay in New York. Because if I hadn't, I would never have met your mother.

So we can assume from this that at this point, he has not yet met the mother. This in turn rules out all the woman up to this point which includes:

• "Krav Maga" Natalie (Sock monkey girl) (S01e04)
• Kelly and Coat Check Girl (S01e05)
• Slutty Pumpkin (S01e06)
• Sara O'Brien from Love Solutions (S01e07)
• Trudy from the pineapple incident (S01e10)
• Mary Beth (S01e11). Ted's date on NYE
• "Cupcake" Victoria (S01e12-e18)
• Mary the paralegal (S01e19)

How about the ladies from Season3? 

• Amy the Tatoo one night stand 
• Jersey girls 
• Soroity sisters Trudy (Again) and Rachel
• Stacey the bass player in the jamacan band 
• Blah Blah girl aka WoW gril
• Cathy ... the girl that talks too much
  
• Ashley the married woman at the st patty's day party 
• Stella 

In Season 4?

• Vicky ... the girl Ted performed the "Naked Man" on
• Karen ... Teds high school gf
• Holli ... the overly desparate one

Hmm OK i think i've mentioned every woman Ted has dated until now. And I think we can rule out all the women he didn't have a serious relationship with (i.e. more than one episode).

So who were the serious ones after Season 2 ep 15? From memory its only Robin and Stella.

We've already ruled out Robin, so what about Stella?

Well Stella is NOT the mother. The story is about how "Ted" met the mother, so if Stella was the mother. STORY OVER! Or at least the kids would think its over. There is also a flash forward scene where the kids are shown with blonde hair if Ted married Stella.

OK so what do we know for sure about the mother???

1. YELLOW Umbrella.  Do I need to cite anything here ... or is it GENERAL KNOWLEDGE *salutes*
2. Architecture Student? Ok I'm assuming here .. but at the end of season 4 ..  when Ted becomes Professor Mosby ...Future Ted says

   I wouldn't have met your mother. Because as you know, she was in that class

3. Somehow ... Stella  plays an important part of Ted meeting the mother. This is clear in Season 4 episode 22 (Right Place, Right Time). The episode centers around Ted being at the right place at the right time ... the right place being at a crosswalk .. the right time being Stella being there too. Ted goes on to say that if he knew that all circumstances would lead him to that place at that time, he would go back and thank everyone. Which implies that this moment is important in the story of Ted meeting the mother.  This is some what validated as Tony (Stella's fiance) is the one that sets Ted up with the gig as a Professor of Architecture. But I'm sure Stella has an even bigger role later on.

OK ...so what about the theories out there .... hold onto your seats =)

Theory 1

The mother's name is ... wait for it .... TRACY (nice yeah?). This was revealed in Season 1 .. Yes Season1 in episode 9. Future Ted joked that this stripper (whose "real" name is Tracy) is the mother. Why would the writers include the name of the stripper as part of the joke ... hmmm ... 

Theory 2

The "Soul Mate" found by Love Solutions. It is also speculated that this is the Mother. Which would make sense in the fact that Ted has yet to met her. Also in the profile it is noted that she shares the same interests as Ted AND wants TWO KIDS ... a BOY and a GIRL. Much like the two kids Future Ted is telling the story to.

Theory 3

Remember the St Patrick's Day Party episode? Ted has said that the mother was there that night. There is a scene where Ted bumps into another girl while walking towards Barney. Ted says sorry and she says "Its OK".  It is also theorized by fans that this might be the Mother, because how else would this brief scene contributed to the story. Some ppl have gone to the extend of freeze framing that moment and trying to match up the clothes of a woman in a earlier scene waiting outside the club with the "yellow umbrella". BUT as it is mentioned, Ted does not meet the mother at the party. So going by that bumping into and acknowledging each other COUNTS as meeting to me.

Theory 4

In Lucky Penny, the episode finishes off by Future Ted saying that his destiny was not to go to Chicago, but to stay in New York. Because if he hadn't, he would have not met the mother. NOW ... why didn't Ted make it to the plane to Chicago on time??? .. well they traced it back to the penny. If Ted didn't find the penny on the subway, then all the resultant chain of events wouldn't of happen to stop him missing his flight. The 1939 penny was also mentioned in earlier in the season in epsiode 2 (Season 2). So maybe the penny has some significance to Ted meeting the mother. When Ted and Robin trace it to Ted finding the penny as the reason why he missed the flight, Robin says " ... unless you know who dropped that penny?" .. Ted replies ... "I wish I did ..."