Star Trek: Next Generation episode "Yesterday's Enterprise" airs 1990

Considered one of the best episodes of Star Trek: The Next Generation, Yesterday's Enterprise, the fifteenth episode of the third season, aired February 17, 1990.  The Enterprise D encounters a rip in the space-time continuum.  The Enterprise C, which had been lost years before, appears, and the present is changed.  The Enterprise D becomes a war ship.  Lt. Tasha Yar appears and Lt. Worf disappears.  Guinan knows something is wrong and demands that Picard send the Enterprise C back, even though it means its imminent destruction. 

The episode won an Emmy Award for Outstanding Sound Editing for a Series. It is the only episode in which Guinan and Tasha Yar have a scene together.  Yar was killed in the first season, and Guinan appeared in the second season.

Earlier drafts of the episode had Worf being the Klingon commander who demands the Enterprise to surrender.  Also, the writers decided to kill Tasha Yar off a second time to give her a more heroic send-off, since her first death was deemed a disappointment. It was the last time Tasha Yar and Wesley Crusher appeared in the same episode. 

Star Trek: The Next Generation episode "Q Who" wins 2 Primetime Emmys

Star Trek: The Next Generation was nominated for eight Primetime Emmy awards, with the episode Q Who receiving three of the nominations.  Q Who won two of the awards at the 41st annual ceremony held September 17, 1989.  Q Who was the first appearance of the Borg.

It won Outstanding Sound Mixing for a Drama Series and Outstanding Sound Editing for a Series. The episode was nominated for Outstanding Achievement in Special Visual Effects.

Episode Elementary Dear Data was nominated for Outstanding Costume Design for a Series and Outstanding Art Direction for a Series.  

Unnatural Selection was nominated for Outstanding Achievement in Hairstyling for a Series.

Episode For the Child was nominated for Outstanding Achievement in Music Composition for a Series (Dramatic Underscore).

Episode For a Matter of Honor was nominated for Outstanding Achievement for Makeup for a Series.

Star Trek: TNG "The Best of Both Worlds, Part 2" wins 2 Primetime Emmys

The conclusion of Star Trek: The Next Generation's cliffhanger from the previous season The Best of Both Worlds, Part 2 won two Primetime Emmys at the 43rd ceremony held August 25, 1991, and was nominated for a third. Five more episodes were received nominations as well.

The first episode of the fourth season had the crew of the Enterprise trying to stop the Borg from destroying Earth and rescue Jean-Luc Picard from their clutches.

Part 2 won for Outstanding Sound Editing for a Series, and Outstanding Sound Mixing for a Drama Series.  It was nominated for Outstanding Achievement in Special Visual Effects along with episode Part 1.

Episodes Brothers and Identity Crisis were nominated for Outstanding Achievement in Makeup for a Series.  The episode Half A Life was nominated for Outstanding Achievement in Music Composition for a Series (Dramatic Underscore). Devil's Due was nominated for Outstanding Costume Design for a Series.

Star Trek: The Next Generation episode "Yesterday's Enterprise" wins Primetime Emmy 1990

Season Three, Episode 15 of Star Trek: The Next Generation, Yesterday's Enterprise, won a Primetime Emmy for Outstanding Sound Editing for a Series at the 42nd ceremony, September 16, 1990.  The episode was also nominated for Outstanding Music Composition for a Series (Dramatic Underscore), and Outstanding Sound Mixing for a Drama Series.  The episode Sins of the Father won a Primetime Emmy for Outstanding Art Direction for a Series.

Four other episodes were nominated: 

Deja Q for Outstanding Editing for a Series

Hollow Pursuits for Outstanding Hairstyling for a series

Allegiance for Outstanding Makeup for a Series

Tin Man, Deja Q for Outstanding Special Visual Effects