This year 60 orchestras and more than 2,000 young musicians will perform across the two cities, travelling from as far as Spain.
The festival, which is organised by the National Association of Youth Orchestras, is now in its 21st year. Each one grows and adds new dimensions: premieres for compositions commissioned for the event, platforms for stars such as Alice Durrant of the Glasgow Schools Symphony Orchestra to perform a double bass concerto specially written for her, and awards for individual orchestras and ensembles.
At the heart of the event are regulars such as the Perth Youth Orchestra, the only group to have performed every year since the festival's inception. Add to them a cluster of new participants each year - several, such as Renfrewshire Schools Percussion Ensemble and East Renfrewshire Schools Symphony Orchestra, are the fruit of government reorganisation - and growing number of foreign groups.
The opening concert this year will be held in Glasgow for the first time. The National Youth Orchestra of Wales and the National Youth Chamber Choir of Wales - 130 musicians in all - will fill the platform with harps and voices in a performance of The Planets (Holst), The Sorcerer's Apprentice (Dukas) and a new commission from Karl Jenkins, Scenes from Wales. It is 45 years since the Welsh (the first national youth orchestra in the world) appeared in Scotland.
All this for pound;80,000, which, NAYO director Carol Main says, is a fantastic achievement but one which could not be realised without the help of long-time sponsors John Lewis, the City of Glasgow and Standard Life.
NAYO has redesigned its website (www.nayo.org.uk) to create direct links with youth orchestras around the country and international links with orchestras around the world. Already, it has received enquiries from Canada, Scandinavia and Slovakia.
Festival of British Youth Orchestras, August 14 - September 3. For Glasgow tickets, tel 0141 332 5057; for Edinburgh, tel 0131 226 5138