Monday 27 February 2017 17:18, UK
After 81 long days without Formula 1 we had 10 faster, meaner, cars back on track in Barcelona - and you can watch them in action here.
Mercedes racked up by far the most laps of the day with Valtteri Bottas completing 79 on his pre-season testing debut in a W08. Lewis Hamilton then added 73 to take the combined total to 152 - with the Brit topping the timesheets for the day in the process.
Ferrari held the fastest lap for much of the day through Sebastian Vettel, as the German impressed in what looks to be a reliable SF70-H, also passing 100 laps.
There were problems for Red Bull in the first session thanks to a sensor issue, with Daniel Ricciardo only completing four laps before lunch as the RB13 was wheeled back to the garage. They were back on track in the afternoon despite a battery issue and he eventually finished fifth after 50 laps.
McLaren had a disappointing day - not for the first time in pre-season testing - as a problems with their Honda engine severely disrupted their running. Fernando Alonso didn't set a timed lap until 3.42pm, and finished 10th.
Armed with a Mercedes engine at a high-speed track, Williams impressed. Felipe Massa even held the fastest lap in the afternoon at one stage and was only three tenths off Hamilton's fastest time.
Force India weren't quite as quick but were similarly competitive, with Sergio Perez behind the wheel of the VJM10 on Monday.
They were much faster than Toro Rosso, who struggled to match their rivals' speed despite completing more laps.
Nico Hulkenberg drove his Renault in an official session for the first time, but ninth place wasn't much to shout home about on his debut.
As for Sauber, Marcus Ericsson wasn't able to lift the new C36 into midfield as the Swiss team were the backrunners on Day One.
The Sky Sports F1 Digital team will be providing live commentary from dawn until dusk on all days of F1 pre-season testing while Sky Sports News HQ will also deliver live updates from trackside.
