Pierre-Emerick Aubameyang's penalty was enough for Arsenal to take all three points against Manchester United on Sunday.
It was Paul Pogba who gave the spot-kick away after 67 minutes when clumsily collided with fullback Hector Bellerin.
Aubameyang's conversion gave Arsenal their first win at Old Trafford since 2006.
Both sides had chances to go ahead in the first-half in what was a high speed start in Manchester.
The Gunners came close through Aubameyang before Mason Greenwood forced Bernd Leno into his first save of the game after an excellent through pass from Marcus Rashford.
Willian then hit bar after a neat one-two with Aubameyang, but the scores remained level heading into the tunnel.
United were ultimately undone by a poor tackle from Pogba which has left them looking up at their rivals in 15th place on the table with just two wins for the season.
"First of all it was a very important win for us when you look at the table," Arsenal goalkeeper Bernd Leno said. "It's been a long time since we won away against a team from the big six, and as well at Old Trafford. It's been 14 years I think, so too long but finally we've done the job.
"It was a very collective, strong performance from us. Our players were brilliant and I think everybody had the right attitude and mentality to win. It was very hard for Man United to create chances.
"At the end of the day, this is the key. The collective performance and defensive work were really important tonight and I think we've worked on that. You can already see that it's much more impressive than it was one or two years ago.
"It's a long, long process and we still have a lot of work ahead of us. We need consistency. This season we've had a lot of good games but also some games where we could have done better. We'll keep working on that though.
"We're all doing a great job and the most important thing is that we stay grounded, we know where we come from and hopefully it goes in the same direction."
