The British Rail Classes 101 and 102 diesel mechanical multiple units were built by Metro-Cammell at Washwood Heath in Birmingham, England from 1956 to 1959, following construction of a series of prototype units. These classes proved to be some of the most successful and longest-lived of BR's First Generation DMUs.

The 101s came in two, three or four car units, with two driving carriages one or two of which were powered by 11.3 litre BUT six cylinder diesel engines with epicyclic gearboxes. Being a first generation DMU, they were a prime example of a slam door train. They had a top speed of 70 mph.

The 101 DMU fleet was vast with 527 101s and 106 of the related Class 102s being built. In all, 760 individual vehicles were built in total. When TOPS was originally introduced only the Driving Motor Brake Second (DMBS) and the Driving Motor Composite (with Lavatory) (DMCL) were classified as Class 101 (AEC engines) or Class 102 (Leyland engines). The Driving Trailer Composite (with Lavatory) (DTCL) were either Class 144 or Class 147. The Trailer Seconds (with Lavatory) (TSL) were Class 162, the Trailer Brake Second (with Lavatory) (TBSL) were Class 168 and the Trailer Composite (with Lavatory) (TCL) were Class 171. Later all the cars were reclassified, becoming Class 101.

The Class 111 was a variant of the 101, having more powerful Rolls-Royce engines.

In 1955, Metropolitan Cammell produced its first lightweight diesel multiple units, the prototypes of what were to become British Rail's most successful and longest-lived First Generation DMU type, the Class 101.

None survived, however in 2024 class 101 DTCL number 56062 was cosmetically converted to represent one of the lost "yellow diamond" Met-Camm lightweight sets. This involved fitting of a lower bufferbeam steel valance, false waist height jumper cable sockets below the cab windows and modified battery box covers. At this point, the vehicle's identity was changed to 79263.

It is planned for class 101 DMBS 51228 to undergo a similar cosmetic conversion whilst out of traffic for bodywork overhaul. This will produce a matching 2-car set.