The Emergency Medical Hologram (EMH) project was started in 2367 by Doctor Lewis Zimmerman to provide additional medical support during times of emergency or when the medical staff aboard ship were unavailable.

A very personal project for Doctor Zimmermann, he used his own physical characteristics to create the original EMH Mark I matrix. The first operational EMH matrix came on-line in late 2369 and the completed EMH Mark I came on-line in early 2371.

The new development was commissioned to be installed aboard the latest vessels and at remote outposts. These included vessels of the Intrepid, Nova and Sovereign classes, with older vessels being retrofitted when and where possible.

In 2373 almost all the EMH Mark I's were taken out of service and reassigned, because of 'flaws' in the EMH's matrix. Of the 676 EMH Mark I's, all but two were reassigned to perform menial tasks. The two missing EMH Mark I's were installed aboard the vessels USS Voyager and USS Equinox. Both these vessels were 'lost' in the Delta quadrant and the Equinox was destroyed in 2374.

The failure of the Mark I EMH and their subsequent reassignment to serve aboard waste transfer barges upset Zimmermann deeply and rather than modify the existing EMH matrix, he started from scratch with a new matrix. This led to the EMH Mark II matrix and through further development of this matrix into the current EMH Mark IV matrix installed throughout the Federation fleet.

Voyager's Mark I EMH was transported back to the Alpha quadrant in 2375 when the first regular contact was established. The Voyager EMH had requested to be transported back to the Alpha quadrant in an attempt to cure his dying creator. It was through the interaction between Doctor Zimmermann and the EMH, with a little help from Counselor Deanna Troi and Lieutenant Reginald Barclay that Doctor Zimmerman realised that the Mark I wasn't a total failure.

The EMH Mark V which is installed aboard the USS Leviathan-A is the latest version of the EMH project and is based directly around the EMH Mark I matrix from USS Voyager. The EMH Mark V has a number of enhancements including the doubling of database storage within the matrix and the introduction of non-essential personality sub-routines.

The most important improvement to the EMH Mark V however is the fact that the whole system is a self-contained unit, including an independent back-up power supply. This means that even if the main computer system is taken off-line, the EMH program can continue to operate.

Finally, the EMH is not confined to the Leviathan-A's sickbay and holodecks as holoemitters are installed in key areas of the vessel, including the main and secondary Bridges, Engineering and all transporter rooms.