Ratification of the International Maritime Organization (IMO) International Convention for the Control and Management of Ships' Ballast Water and Sediments (BWMC)[i] has now forced the hand of operators and ship owners to comply with the BWM, and without a great deal of time to react.  The need to integrate and operate Ballast Water Treatment (BWT) Systems on In-Service ships, however, is not new, and BMT have recent and relevant experience in end-to-end BWT system solutions. In 2011 BMT Defence Services Ltd (BMT) produced the installation specification to enable A&P Falmouth Ltd (A&P) to carry out a ballast water treatment plant retrofit installation on the RFA Bay Class ships (i.e. RFA MOUNTS BAY, LARGS BAY and CARDIGAN BAY), enabling the ship to be approved and operated in compliance with the BWMC. In addition, BMT produced documentation to enable A&P to arrange, manage and perform the necessary test, trials and commissioning to prove the equipment installation and that it can be operated in accordance with the requirements of the BWMC. The objective of this paper is to take the reader through the process of design and embodiment of a BWT system retrofit on a relatively complex (with respect to the ballast system) ship.  The Bay Class ships are Landing Ship Dock (Auxiliary) ships with separate forward and aft ballast systems (to allow the ships to function in their amphibious roles by way of their stern dock), and a ship wide Ballast Stripping system.  Key design and engineering considerations are discussed below and recommendations for the installation of a ballast water treatment system are also offered. International Convention for the Control and Management of Ships' Ballast Water and Sediments, International Maritime Organization, Adoption: 13 February 2004; Entry into force: 8 September 2017.