The Enviro400 chassis replaced the Dennis Trident 2 (and continued to be badged as the Trident for a time) and was formerly available with East Lancs Myllennium Lolyne and Optare Olympus bodywork; the Enviro400 body replaced the Alexander ALX400 designs and was sold on Scania N230UD and Volvo B9TL chassis.

Alexander Dennis released the second generation Enviro400 at Euro Bus Expo in 2008, with the main external visual differences being the incorporation of a line of white LED daytime running lights underneath the headlights in the main front light clusters, redesigned front bumpers and the relocation of the offside emergency exit door.

The second generation Enviro400 was superseded by the Enviro400 MMC in 2015 in the UK with no more orders, but in 2016 Alexander Dennis developed a low-height Enviro400 with Enviro400 MMC-style front and rear specifically for Hong Kong market.

It is also popular with Stagecoach, which placed its first order of 130 low-height models in February 2006, later followed by another 389 buses with Euro IV/V engines.

A much larger order of 128 Enviro400s was placed by Arriva Merseyside division from the onwards, with the first 28 arriving in the autumn of 2014 and the remainder being delivered between January and July 2015 to Bootle, Green Lane, St Helens, Southport and Speke depots.

Arriva Buses Wales, meanwhile, received a total of 29 high specification Enviro400s between June 2014 and March 2015 for deployment on two new Sapphire services serving Rhyl to Llandudno and Chester to Connah's Quay respectively.

[8] The FirstGroup received a large intake of Enviro400s ahead of both the 2012 Summer Olympics and the 2014 Commonwealth Games, which were held in London and Glasgow respectively.

[11] In 2008, a specially converted Enviro400 was assembled in knock-down kit form for the closing ceremonies of the 2008 Summer Olympics and 2008 Paralympic Games in Beijing.

[25] Other operators who purchased Enviro400Hs included National Express West Midlands and National Express Dundee, ordering a total of nineteen for service in Birmingham and Dundee in 2013;[26][27] East Yorkshire Motor Services, who purchased ten for cross-city services in Kingston upon Hull in 2011;[28][29] Reading Buses, who had 31 Enviro400Hs delivered between 2010 and 2011;[30][29] Lothian Buses, who took on 15 of the type in 2011;[31] the Oxford Bus Company, who took on examples to upgrade its park & ride fleet,[32] and First Glasgow.

These Scania Enviro400s were delivered with audio-visual next stop announcements, mood lighting and leather seating.

[44][45] The Oxford Bus Company received eleven high-specification Scania Enviro400s in 2009 for service on the Brookesbus network.

[47] On 13 October 2006, Solent Blue Line received a Volvo B7TL/Enviro400 meeting the Euro III emission standard, with the interior in Wilts & Dorset livery.

On 11 November 2010, Alexander Dennis announced it had received an order for 10 Enviro400s from Coach USA subsidiary Twin America, in New York.