The Alien franchise returns with its first proper installment in 20 years with Alien: Covenant. The film is technically the first Alien movie (not counting Prometheus) directed by Ridley Scott since the original Alien from 1979, and it promises to bring the long-running franchise back to its terrifying horror movie roots while showing how the xenomorphs — the alien monsters from the series — were first created. So since the movie is a prequel to the franchise, does that mean you should see the Alien movies before Covenant?
As previously stated, the Alien franchise has been around since 1979, and including spinoffs is made up of seven films. The proper series consists of 1979's Alien, its 1986 sequel Aliens, 1992's Alien 3, and 1997's Alien: Resurrection. There are also two crossover/spinoffs with the Predator franchise in 2004's Alien vs. Predator and 2007's Aliens vs. Predator: Requiem. And of course, there is the pseudo-prequel/spinoff Prometheus, which was directed by Scott as a way to begin his origin story for the Alien franchise while technically being outside the proper franchise. So that's a full seven Alien movies already out there before Covenant hits theaters, but do you really have to watch all of them to get caught up?
Nah. Right of the bat you can skip the two Predator crossovers, as they're not relevant to the rest of the franchise. That leaves you with the four Alien films, plus Prometheus. And if you're only going to watch one of these movies before Covenant, you should make it Prometheus. Covenant is essentially a sequel to Prometheus and a prequel to Alien, but its timeframe is much closer to that of Prometheus. It features the planet of the Engineers from Prometheus, and stars Michael Fassbender as David, the villainous android from that movie who also plays a key role in Covenant. So without a doubt, Prometheus is a must-watch before Covenant. But what about the rest?
You can definitely get away with skipping Alien 3 and Resurrection, and probably Aliens, but I would recommend that you at least watch Alien. Even though Covenant is expected to be followed by another two or three sequels before catching up to the events of Alien, it's still a prequel, and it would behoove you to familiarize yourself with that movie's original concepts; such as xenomorphs and facehuggers. Plus, it's the only other film in the franchise besides Prometheus that's directed by Scott, so you can expect him to reference it fairly heavily.
And even though you don't technically need to see Aliens to understand Covenant, I'd still recommend watching it if for no other reason than to see Sigourney Weaver kick a bunch of alien butt. It's maybe the best example of a female-led action movie ever made — which is somewhat disappointing since it was released over three decades ago and has yet to be topped — but Covenant at least makes an attempt with another badass lead in Katherine Waterson. And because of her, the new film is well-worth watching regardless of your previous experience with the Alien franchise.