Assembly line: industrial arrangement of machines, equipment, and workers for continuous flow of workpieces in mass-production operations.

An assembly line is designed by determining the sequences of operations for manufacture of each product component as well as the final product. Each movement of material is made as simple and short as possible with no cross flow or backtracking. Work assignments, numbers of machines, and production rates are programmed so that all operations performed along the line are compatible.

An automotive assembly line starts with a bare chassis; components are attached successively as the growing assemblage moves along a conveyor. Parts are matched into subassemblies on feeder lines that intersect the main line to deliver body parts, engines, and other assemblies. As the units move past, each worker along the line performs a specific function. Each part and tool is delivered to its point of use in synchronization with the line. A number of different assemblies are on the line simultaneously, but an intricate system of scheduling and control ensures that the appropriate body type and colour, trim, engine, and optional equipment arrive together to make the desired combinations.

Automated assembly lines consist entirely of machines run by machines. In such continuous-process industries as petroleum refining and chemical manufacture and in many modern automobile-engine plants, assembly lines are completely mechanized and consist almost entirely of automatic, self-regulating equipment.

Most products, however, are still assembled by hand because many component parts are not easily handled by a simple mechanism. The number of products automatically assembled is steadily increasing but at a low rate because a product must be designed for automatic assembly and must be accurately and consistently manufactured. Expensive and somewhat inflexible, automatic assembly machines are economical only if run at very high outputs. However, the development of versatile automatic machinery and industrial robots is increasing the flexibility of fully automated assembly operations.