More than 2000 runners from 48 countries will take part in the Mattherhorn Ultraks next week in Switzerland.
In the shadow of the Matterhorn, one of the most photographed summits in the world, runners of all abilities will line up in Zermatt for the fifth edition.
American Megan Kimmel is back to defend her title in the 46km race, which involves 3600m of ascent on a course so tough that Kilian Jornet's course record is 4 hours 43 minutes.
She will face competition from Netherlands' Ragna Debats, who is overall leader in the Skyrunner World Series, of which this event is part.
Men's series leader Aritz Egea of Spain heads the men's 46km.
New Zealander Ruth Croft tries to defend her 30k title, while a 16km has also been added this year.
This year the race will award 20% more points for Sky Classic rankings and the course has been adapted to cater for a record entry level.