The A40 is a major trunk road connecting London to Fishguard, Wales. It is 256 miles long.

It is one of the few remaining of the "old" trunk routes not to have been superseded by a direct motorway link, though some parts, such as the southern section from London to Oxford are served by the M40. Edward Platt authored a book entitled Leadville, an intriguing mix of sociology and town planning, and a record of how the A40 was conceived and how it shaped and continues to influence those who live beside and travel on it.

Part of the A40 forms a section of the unsigned Euroroute E30.