Semi-automatic transmission, or also known as clutchless manual transmission or automated manual transmission, is a system which uses electronic sensors, processors and actuators to do gear shifts on the command of the driver. This removes the need for a clutch pedal which the driver otherwise needs to depress before making a gear change, since the clutch itself is actuated by electronic equipment which can synchronise the timing and torque required to make gear shifts quick and smooth. The system was designed by European automobile manufacturers to provide a better driving experience, especially in cities where congestion frequently causes stop-and-go traffic patterns.

In standard mass-production automobiles, the gear lever appears similar to manual shifts, except that the gear stick only moves forward and backward to shift into higher and lower gears, instead of the traditional H-pattern. The Bugatti Veyron uses this approach for its 7-speed transmission. In Formula One, the system is adapted to fit onto the steering wheel in the form of two paddles; depressing the right paddle shifts into a higher gear, while depressing the left paddle shifts into a lower one. Numerous road cars have inherited the same mechanism.

Hall effect sensors sense the direction of requested shift, and this input, together with a sensor in the gear box which senses the current speed and gear selected, feeds into a central processing unit. This unit then determines the optimal timing and torque required for a smooth clutch engagement, based on input from these two sensors as well as other factors, such as engine rotation, the Electronic Stability Program, air conditioner and dashboard instruments.

The central processing unit powers a hydro-mechanical unit to either engage or disengage the clutch, which is kept in close synchronization with the gear-shifting action the driver has started. The hydro-mechanical unit contains a servomotor coupled to a gear arrangement for a linear actuator, which uses brake fluid from the braking system to impel a hydraulic cylinder to move the main clutch actuator.

The power of the system lies in the fact that electronic equipment can react much faster and more precisely than a human, and takes advantage of the precision of electronic signals to allow a complete clutch operation without the intervention of the driver.

Historically, the first semi-automatic transmission which was marketed was the 1941 M4/Vacamatic Transmission by Chrysler. It was an early attempt at an automatic transmission that still required the use of a clutch, primarily to start and stop. Later, the Volkswagen Beetle came with an optional "Autostick", which was essentially a clutchless manual with three forward gears.

Citroën is one manufacturer that committed to semi automatic transmission. First appearing in 1955 on the Citroën DS, a hydraulic system was used to select gears and operate the conventional clutch using hydraulic servos. There was also a speed controller and idle speed step-up device, all hydraulically operated. This was a clutchless shifting with a single column mounted selector. The Citroën 2CV gained a device named 'Trafficlutch', a centrifugal clutch that enabled clutchless changes in the first two ratios only (for town driving). The DS's semi automatic transmission was nicknamed 'Citro-Matic' in the United States. Later, the manufacturer introduced optional semi automatic transmissions on their medium and large saloon and estate models in the 1970s; the Citroën GS and CX models had the option of 3 speed, semiautomatic transmission marketed as 'C matic'. This was simpler than the previous inasmuch as it used a floor mounted quadrant lever operating a contact breaker and conventional gear selector rods in series, a fluid coupling 'torque convertor' and wet plate clutch were cut in and out of phase by an electro valve controlled by the contact breaker. This system was simple in that it dispensed with the former use of hydraulics to operate a clutch AND select the gear ratios. Citroën semi automatic transmission of this era made no use of electronics, the entire gear selecting operation was carried out by simply moving the gear lever from one ratio to the next.

The German automobile manufacturer, NSU, produced a semi automatic system for the rotary engined Ro80 saloon car in the 1960s, similar in concept to Citroën's system except that it used an electric switch on the gear shifter which disengaged the clutch.

The 993cc Diahatsu Charade in 1985 at least, had the option of a 2-speed semi-automatic transmission which was similar to a conventional auto with torque converter and planetary gearset but lacked a full valve body for making decisions regarding shifting. This was left entirely to the driver and as a result could be accelerated from rest in top gear if desired, depending entirely on the torque converter action. This was not as bad as might seem; standing quarter mile time with two 60kg occupants and using low gear appropriately was 21.0 sec while using top gear only was 21.5 sec.

Drag Racers have their own type of clutchless manual transmissions. A Liberty is basically a manual transmission with no clutch, and is used in Pro Stock. The Lenco is a transmission also used in drag racers. A Lenco is different from a Liberty because a Lenco uses planetary gears, like an automatic. Both transmissions can be manually shifted, or use an air shifter. However, they require a clutch to use when leaving off the line for traction. The Lenco uses separate levers to shift while the Liberty uses a single shifter hooked up to several levers. A variation of the Lenco called a Lencodrive utilizes a torque converter and no clutch.

Some variants of the system, such as Citroën's Sensodrive, allow the driver to select automatic mode, in which the processor takes responsibility for gear changes. The car then drives much like a standard automatic, including features such as kickdown, but with fuel consumption pretty similar to a manual. Gears can be selected using either stick or paddles, both in manual control and as a temporary change in automatic mode (for example before starting to overtake another car).

Semi-automatic transmissions have also made its way into the truck and bus market in the early 2000s. Volvo offers its i-Shift on its heavier trucks and buses, while ZF Friedrichshafen AG markets its ASTronic system for buses and coaches. These gearboxes have a place in public transport as they have been shown to significantly reduce fuel consumption.

In the UK though, semi-automatic transmission has been very popular on buses for some time, from the 1950's right through to the 1980's, an example being the well known London Routemaster, although the latter could also be driven as a fully automatic in the 3 highest gears. Leyland manufactured many buses with semi-automatic transmission, including it's Leopard and Tiger coaches. Fully automatic transmission became popular with increasing numbers of continental buses being bought in the UK, and more and more British manufacturers began offering automatic options, mostly using imported gearboxes, and semi-automatic transmission lost favour. These days, very few buses with semi-automatic transmission remain in service, although many are still on the roads with private owners. Modern types of semi-automatic transmission though is becoming more common, mostly replacing manual gearboxes in coaches and small buses.