(written from a Production point of view)
Star Trek/Planet of the Apes: The Primate Directive is a miniseries produced by IDW Publishing in conjunction with BOOM! Studios. The series features the Original Series characters teaming up with the original characters from the first Planet of the Apes series.
According to David Tipton, the plot involves "The Klingons secretly backing a renegade gorilla general in a coup for control of Ape City, Captain Kirk finds himself in the uncomfortable position of having to help out Dr. Zaius' orangutans. Taylor won't be happy with that!"
IDW editor-in-chief Chris Ryall mentioned the notion of a Star Trek/Planet of the Apes crossover to the Tiptons following the success of Star Trek: The Next Generation - Doctor Who: Assimilation². They were puzzled by the idea of gelling together two disparate visions of the future, but realized as alternate timelines were part of the fabric of Star Trek, it was worth attempting. They opted to crossover the original versions of both series, as these were deemed the most iconic, and because of their roots in the culture of the 1960s. Scott Tipton noted it was appropriate that John Chambers' designs for the Apes would be depicted alongside Spock. 
- Kirk And... Those Damn Dirty Apes? at StarTrek.com, the official Star Trek website
- Tiptons, Scott Map Out the "Star Trek/Planet Of The Apes" Crossover
- Star Trek—Planet of the Apes at Memory Beta, the wiki for licensed Star Trek works
- Star Trek/Planet of the Apes: The Primate Directive at The Sacred Scrolls