The campaign medal rolls are a vital source of information, but the degree of detail they include varies.
What are they?
The rolls are large, heavy, bound volumes. The originals are held at the National Archives (collection reference WO329). They were compiled by the Army Medals Offices as a record of issue of the various campaign medals, mainly during the period 1919-1922.
The volumes are organised by type of medal and by regiment. Each contains details of thousands of issued medals. An index card system (“medal index card”) was created to enable a man to be located in a given volume and on a given page. Until digitisation, this was the only practical way for locating an entry: now, the details can be searched relatively easily and the entry found without reference to the index card.
Where are they?
Ancestry and its associated (mainly USA) site Fold3 are the only online places to see images of the original rolls. The examples shown on this page are all screen clips from the rolls at Ancestry.
NMArchive (the online service of Naval & Military Press) has transcripts of the details from the rolls.
At present, no other provider offers details from the rolls.
What do the rolls show?
Especially if a man’s service record cannot be located, the rolls are a vital source of information.
Using examples of soldiers named John Smith, the variation in the detail included in the rolls can be seen below.
Let’s begin with the most common roll; that of the issue of the pair of the British War Medal and Victory Medal.
Example 1: London Regiment and Royal Fusiliers (City of London Regiment)
If your soldier served with either of these regiments then you are in luck.

The roll gives each man’s number(s), rank, unit(s) and – most helpfully – the dates of entry and exit of the theatres of war, which are referred to by codes (in this) example, 1a, 2a and 4b). The last date is given as 11 November 1918 simply as this was when hostilities ceased in France and Flanders. If a man’s final exit date is earlier it usually implies that he had lost his life or had left due to wound or illness.
Note that the page is headed 20th [Battalion of the ] London Regiment. From his original unit was created the 1/20th and 2/20th Battalions, both of which appear on the page.
Note that every man appears under two numbers: for example, John Henry Smith is shown as 5089 and 632280. This relates to the fact that the 20th London was unit of the Territorial Force and its men were renumbered in 1917.
Example 2: Other infantry regiments
Here is an example entry for a John William Smith of the Wiltshire Regiment.

The entry is less detailed that those of the London Regiment/Royal Fusiliers rolls, in that no dates or theatres of war are given.
The man’s number(s), rank, regiment(s), unit(s) are given, and in this example certain useful information is added: transferred to reserve (in the case of Attwell and Lay) or discharge (Smith was “no longer physically fit”), with dates.
Most rolls for the infantry regiments do not include that useful added detail. This roll of the North Staffordshire Regiment is typical in that it provides only the number(s), rank and unit(s).

The units that are listed are generally in chronological order although I do sometimes find errors have been made in that regard. The units should not (in theory) include any with which the man only served at home before he went overseas.
Example 3: the larger corps – artillery, engineers and others
The rolls for the larger corps generally provide no information about the man’s unit(s) or given any useful dates.

This roll of men who were transferred into the Labour Corps is typical and provides only the barest information.
Note the pencil ticks and initialling. They are believed to be connected to the issuing of the medals but all meaning of the specific initials and letters that are sometimes seen has long since been lost.
Example 4: the roll of the 1914 Star
The roll for this medal is helpful for those men serving with the larger corps, for it provides details of the unit with which they were serving when they first went overseas.

Note that the men’s unit is given at the top of the page – in this instance, 25th Brigade of the Royal Field Artillery.
Links
Interpreting campaign medal index cards
The theatre of war codes used in medal rolls and index cards