The EH 101 Merlin is a joint design by Westland (UK) and Agusta (Italy) which formed the bi-national company EH Industries, now absorbed into AgustaWestland after the merger of these two firms. Born from similar requirements from the UK's Ministry of Defense and Italy's Marina Militare as a replacement for the ageing Wessex and Sea Kings, the EH101 is a medium-lift helicopter designed for both military and civil use and is roughly equivalent to the Super Puma in size and layout albeit with a three-turbine powerplant and a five-blade main rotor. Its main roles are as a medium transport capable of carrying as many as 30 fully equipped troops and a navalized anti-submarine warfare platform equipped with a wide variety of sophisticated avionics and systems including a glass cockpit and FLIR plus a weapons load composed of torpedoes, depth charges, or anti-ship missiles. Besides the UK and Italy, foreign operators include Canada, Denmark, Japan, Portugal, and the United States.

The Merlin first flew on 9 October 1987 but problems during development (including a number of crashes) led to service introduction only until a decade later with the Royal Navy as the Merlin HM.1 ASW platform which was capable of operating from frigates and Invincible-class carriers. These were followed by the Merlin HC.3 transport helicopters for the RAF and differs by having a double-wheel landing gear. Other versions are generally referred to simply as the EH 101 aside from a number of redesignated foreign versions such as the Canadian CH-149 Cormorant and the US VIP transport known as the US101, later redesignated VH-71 Kestrel which is scheduled to replace the USMC's current Marine One presidential transport fleet.