Brentford held on for their third Premier League win of the season as goals from Ethan Pinnock and Bryan Mbeumo sank a wasteful Chelsea in the West London derby at Stamford Bridge.
The Blues, who missed several chances, were punished through Pinnock's thumping header from Mbeumo's wicked cross.
With Chelsea goalkeeper Robert Sanchez up the field for a late corner, Neal Maupay led a Brentford counter-attack and set up Mbeumo for the Bees' second.
Chelsea, who went closest in the first half when Noni Madueke struck the woodwork, slipped to a ninth home defeat this year thanks to some poor finishing.
Brentford keeper Mark Flekken was never truly stretched and they defended resolutely before hitting Chelsea on the break.
Brilliant Bees sting Chelsea
Brentford have won on all three of their visits to Stamford Bridge as a Premier League club, and, although Chelsea was largely the better side, Brentford still created their fair share of chances.
Mathias Jensen was given room down the right just after the restart, cutting the ball back to Vitaly Janelt, who guided his shot straight at Sanchez.
Brentford's goal was created down the right when Mbeumo fired in a brilliant, pacy delivery and Pinnock sprinted to meet it with a powerful header.
On the break, Yegor Yarmolyuk was clean through when he was kept out by Sanchez before Mbeumo stabbed a shot narrowly wide.
Thomas Frank's side added a second when Mbeumo tapped the ball into an empty net with only Sanchez nearby.
After a slow start to the season, Brentford have now won two games in succession while extending their unbeaten run in London derbies to 13 matches.