Season One, Episode Six - Penny kisses Leonard for the first time.
Season One, Episode Seventeen - Leonard asks Penny out. She says yes.
Season Two, Episode Fifteen - Leonard and Penny have their first make-out, taking it as far as the bedroom before the mood was ruined.
Season Two, Episode Twenty-three - Penny's feeling for Leonard are voiced to the audience for the first time.
Season Three, Episode One - Leonard and Penny sleep together for the first time.
Season Three, Episode Seven - Leonard and Penny survive their first fight as a couple.
Season Three, Episode Nineteen - Leonard tells Penny that he loves her.
Season Four, Episode Twenty-four - Penny admits that she should never have broken up with Leonard.
Season One, Episode Seventeen - Leonard asks Penny out. She says yes.
Season Two, Episode Fifteen - Leonard and Penny have their first make-out, taking it as far as the bedroom before the mood was ruined.
Season Two, Episode Twenty-three - Penny's feeling for Leonard are voiced to the audience for the first time.
Season Three, Episode One - Leonard and Penny sleep together for the first time.
Season Three, Episode Seven - Leonard and Penny survive their first fight as a couple.
Season Three, Episode Nineteen - Leonard tells Penny that he loves her.
Season Four, Episode Twenty-four - Penny admits that she should never have broken up with Leonard.