From Edinburgh, the A90 travels west and over the Forth Road Bridge, before turning into the M90 motorway. At Perth, the M90 again becomes the A90, now running north east to Dundee and through the Kingsway road system. It then passes Forfar, Brechin, Stracathro (the site of an ancient Roman Camp), Stonehaven, Bridge of Muchalls (where the Burn of Muchalls flows under), near Muchalls Castle, near Saint Ternan's Church, Newtonhill, Portlethen, from there through the city of Aberdeen, crossing the Ythan Estuary, on to Peterhead on its way to Fraserburgh. The A952 road serves as a more direct inland bypass to the A90 road in its northerly extent.
The Dundee to Aberdeen stretch of the A90 is notorious for speed cameras. Unusually, the 60-mile (97 km) trip from Dundee to Aberdeen along the A90 entails over a dozen speed cameras including a majority of fixed Gatso types as well as locations used by mobile camera vans. These cameras can be found on long fast stretches of road, and shortly before dangerous junctions, such as at the Laurencekirk junction where a 50 mph (80 km/h) speed limit is in force. This was introduced due to this junction's appalling safety record. A similar speed restriction was imposed at Forfar until two new grade-separated junctions were built, after which the restriction was lifted.
The A90 was extended considerably on 30 March 1994 so as to give a continuous route number from Edinburgh to Aberdeen, since the A90 originally stopped at Inverkeithing. The former A85 went from Perth to Dundee; the A929 Kingsway through Dundee and then towards Forfar; the A94 from Forfar to Stonehaven; and the A92 from Stonehaven to Fraserburgh. The A92 did not go to Peterhead. The evidence of the numbering change can still be seen on the older road signs along the route where the original route numbers have been covered over with "A90" plates.
A short stretch of the A90, from the southern terminus of the M90 to Barnton Junction (a junction with the A902), is part of Euroroute E15, which runs from Inverness to Algeciras in Spain. The E15 continues northwards on the M90, and southwards on the A902 leading to the Edinburgh City Bypass.