Ross dating janice
Season 5 is when Janice shows up again - this time as Ross' hook-up in The One With Chandler's Work Laugh.
Ross, who is currently divorced twice, evicted and unemployed, has just learned that his second ex-wife Emily is getting married again, and in despair he goes out and stays out all night, running into Janice and hooking up with her.
He sleeps with her and then dumps her again the next day, which happens to be Valentine's Day.
In the beginning of Season 2, a lonely Chandler who is afraid of becoming like Mr Heckles decides to call Janice, but she reveals herself at Central Perk, married and pregnant.
Season 8 is almost entirely in the absence of Janice, except for the finale (parts 1 and 2).
Only in the last episode of Season 2 does Chandler hook up with her again, after flirting with her on the Internet (although he does not know her identity during the conversations, he does discover that she's married).
This starts his longest relationship with Janice during the course of the show, which ends in The One With The Giant Poking Device, when Chandler finds out she's cheating on him with her soon-to-be ex-husband and tells her to try and re-construct her family.
She is known for her irritating, nasally voice and her trademark cry of "OH... Later that season, Chandler invites her to a New Year's party in a desperate attempt to have someone to kiss, but can't stand being around her and breaks up with her again before midnight, leaving him so cranky that Joey ends up kissing him.
Later in the first season, Chandler and Janice hook up again after Joey unknowingly sets Chandler up with her on a double-date.
In Season 9, Janice shows up at a fertility clinic in The One With The Fertility Test.