There goes the final whistle. It has ended 0-0. The second leg is next Wednesday at the Emirates.
Besiktas manager Slaven Bilic has been sent to the stands for complaining.
FULL TIME: Napoli 1-1 Athletic Bilbao
Oxlade-Chamberlain has hit the post. So unfortunate. Brilliant little run and finish, but the ball comes back out. Almost a dramatic late winner.
Athletic sub: Ander Iturraspe OFF, Erik Moran ON.
Athletic are almost undone by a defensive mix-up. Gurpegi leaves a ball over the top for his goalkeeper, unaware that Higuain is lurking behind him, but the Napoli striker puts his shot wide from a tight angle.
Incredible save! Mertens drops a shoulder and bursts into the box, squaring it back to Higuain. The striker powers a shot towards goal but Iraizoz sticks a hand up to make the stop!
Oguzhan Ozyakup is going to depart the scene with Kerim Koyunlu taking over.
Good effort, good save! Mertens drives forward down the left, cutting inside onto his right foot, but his shot is saved on the stretch by Iraizoz. It was a comfortable enough save, but good strike nonetheless.
A second yellow card for Ramsey, and he is off the park. Red card for Ramsey after taking out Oguzhan Ozyakup.
Monreal drives the free-kick through the wall, but straight into the arms of Zengin, who manages to collect the loose ball at the second attempt.
What was Michu doing! A cross from the byline falls at the feet of Michu eight yards out who has a fantastic chance to score. But incredibly he passes backwards. I can't believe he's done that, and neither can the San Paolo crowd!
Oguzhan Ozyakup goes into the book. This match all a bit messy and ragged with neither side creating much.
Athletic sub: Markel Susaeta OFF, Ibai Gomez ON.
Napoli sub: Marek Hamsik OFF, Michu ON.
YELLOW CARD: Mikel Balenziaga (Athletic Bilbao)
On comes Tore for Besiktas, off goes Sahan.
Athletic sub: Benat OFF, Mikel San Jose ON.
Sanchez departs the scene to be replaced by Alex Oxlade-Chamberlain for the visiting team.
What a chance! Callejon beats the offside trap down the right channel, but after faking the shot and making the keeper go down he puts his shot well wide. That was a sitter! Should have scored.
Sanchez to Debuchy, ball comes across the face of the Besiktas goal, but nobody on hand to convert. Real chance for Arsenal.
Monreal goes in late on Pektemek. And he is going to be booked for that one. Can have no complaints.
GOAL! Napoli 1 (69 mins - Gonzalo Higuain) Athletic Bilbao 1. Out of absolutely nothing Higuain has pulled his team level! He tip-toes his way through the box before hitting hard and low into the bottom corner!
Ramsey picked up his booking after taking out Necip Uysal. Bit silly from him. The home fans again making plenty of noise.
Napoli sub: Lorenzo Insigne OFF, Dries Mertens ON.
Aaron Ramsey has gone into the book for a late challenge, and Flamini then brings down Kavlak. Referee Milorad Mazic suddenly busy.
Motta has picked up a booking for a late, late challenge on Sanchez as he went to ground.
Glorious chance for Sahan, but he knocks the ball wide of goal after managing to turn inside Koscielny, who again gets beaten far too easily. Poor, poor defending really.
What a chance for Athletic! Rico shoots on goal from the edge of the box, but Aduriz tries to deflect it goalwards on the first time turn but he puts it wide. He was onside as well! What a chance.
Off we go then for the second period. Arsenal get it off and running.
The second half is under way between Napoli and Athletic Bilbao!
Lovely pass from Ramsey to find Giroud, but he can't cash in as the ball drops into the arms of Zengin.
HALF TIME: Napoli 0-1 Athletic Bilbao. The Italians started the game strong but Athletic have since grown into the contest and have turned in the perfect away performance in the Champions League.
Error by Chambers, but Ba can't punish the youngster. Bounds in towards goal, but drives the shot wide from 16 yards with Szczesny diving to his left. Sholud have been the opener.
GOAL! Napoli 0 Athletic Bilbao (42 mins - Iker Munian). What a huge goal! De Marcos makes a burst from right back before squaring the ball for Muniain, who picks his spot from 12 yards. Excellent goal.
Good opportunity for Athletic! Susaeta gets to the byline and digs a cross out, with Aduriz leaping highest in the penalty area. But from six yards out he can't get enough power in his effort and Cabral saves.
He should have at least hit the target! What an opportunity for Bilbao. The corner kick is played into the box from the right side for Laporte, who is unmarked in the penalty area. But he manages to direct his header wide!
Sloppy defending by Besiktas, but Cazorla drives the ball wide of goal from the Sanchez pass. Decent hit and run, but just off target as the shot sails wide.
Giroud with a chance at the other end, but he can't turn in time to stab the ball home from the Sanchez ball. Zengin eventually picks up. Decent enough defending by Besiktas.
Szczesny with a fine stop from a Ba volley. Ozyakup with the cross. Superb little delivery from him.. Fine attacking move.
YELLOW CARD: Jorginho (Napoli)
Another decent chance for Napoli, who have made a very good start to this game. This time it's Hamsik who puts his shot wide of the target. He should have done better from there.
And the home side hit the bar. Demba Bar hits the bar from the halfway line. Not sure Szczesny was getting there. So unfortunate.
The first chance of the game falls to Napoli, with Higuain squaring the ball back from the by-line. But Callejon misjudges his leap and directs his header over the bar. Decent opportunity.
Off we go then with Besiktas taking the kick-off. The home side get the ball rolling.
We're under way at the San Paolo! It's Napoli v Athletic Club!
Just to confirm both starting teams for you this evening: Besiktas: Tolga Zengin, Ismail Koybasi, Ramon Motta, Ersan Gulum, Pedro Franco, Olcay Sahan, Veli Kavlak, Necip Uysal, Oğuzhan Ozyakup, Mustafa Pektemek, Demba Ba.
Arsenal: Wojciech Szczesny, Mathieu Debuchy, Calum Chambers, Laurent Koscielny, Nacho Monreal, Mikel Arteta, Jack Wilshere, Aaron Ramsey, Alexis Sanchez, Santi Cazorla, Olivier Giroud
TEAM NEWS: Napoli - Rafael, Maggio, Albiol, Koulibaly, Britos, Jorginho, Gargano, Insigne, Hamsik, Callejon, Higuain. /// Athletic - Iraizoz, De Marcos, Gurpegui, Laporte, Balentziaga, Iturraspe, Rico, Benat, Susaeta, Muniain, Aduriz.