Med kits are stored in bulk on board starships and are primarily used on away missions and during emergency relief aid, such as helping populations suffering from viral outbreaks or epidemics.
The most important piece of equipment in the med kit is the medical tricorder. This device scans the patient to find the extent of the injury or the type of disease. Other devices normally contained within the kit include a respirator, a field hypospray, drugs, and a defibrillator module. Each kit can be modified for a particular mission by taking the items that are most likely to be needed.
Starfleet equipment is always being updated and improved, and medical kits are no exception. In the early to mid 2360's medical kits were carried by a handle and split into halves like a briefcase.
In the late 2360's the med kit was redesigned so that it now looks more like a rucksack and is carried over the shoulder by a single strap. The other major change was that rather than having to open the case to find the tricorder, it now slides out of a specialized compartment.
This is the most common med kit in use, but in 2374 Dr. Bashir used an alternate version that was made of cloth and looked more like a satchel. It, too, was carried over the shoulder, and held a number of essential medical devices.