As the name suggests, the original function of a millwright was the construction of flour mills, sawmills, paper mills and fulling mills powered by water or wind, mostly of wood with a limited number of metal parts.. Since both of these structures originated from antiquity, millwrighting could be considered, arguably, as one of the oldest engineering trades and the forerunner of the modern mechanical engineer. 
In modern usage, a millwright is engaged with the erection of machinery. This includes such tasks as leveling, aligning and installing machinery on foundations or base plates and setting, leveling and aligning electric motors or other power sources such as turbines with the equipment, which millwrights typically connect with some type of coupling. Related but distinctly different crafts include machinists and mechanics.
The exact duties of a millwright vary depending on whether they are unionized or not, with union rules typically being more restrictive than non-union situations, which may have their own job description.
Pre Modern Era
Originally, millwrights were specialized carpenters who completely designed and constructed mills. Having a working knowledge of driveshafts, bearings, gearing and mechanical belts, they executed every type of engineering operation in the construction of these mills. They designed the patterns of the water wheel systems, carved their gear mechanisms, and finally erected the mill machines 
In the Hellenistic period Greek millwrights invented the two main components of watermills, the waterwheel and toothed gearing. Greeks, along with the Romans, were the first to operate undershot, overshot and breastshot waterwheel mills.
Muslim millwrights adopted the Greek watermill technology from the Byzantine Empire, where it had been applied for centuries in those provinces conquered by the Muslims. They used several solutions to achieve the maximum output from a watermill, by either mounting them to piers of bridges to take advantage of the increased flow or by using a shipmill, a type of watermill powered by water wheels mounted on the sides of ships moored in midstream.
In Medieval Europe, millwrights built the first industrial mills which introduced new innovative uses of waterpower. A survey of the types introduced in Western Europe is conducted by Adam Robert Lucas. 
In China, in late 14th century, the millwrights were known as jiang and kong (a special term for artisan-engineers) and existed at an early age. They learned their craft on the shop floor, in a kind of apprentiship scheme. .
Sir William Fairbairn, a millwright of the late 19th Century, wrote in his Treatise on Mills and Millwork, "...the millwright of the late centuries was an itinerant engineer and mechanic of high reputation. He could handle the axe, the hammer, and the plane with equal skill and precision...he could set out and cut in the furrows of a millstone with an accuracy equal of superior to that of the miller himself." 
The introduction of the steam engine and the increasingly importance of iron and steel changed the global industrial landscape. It created specialisation and the birth of new trades ( Turners, fitters, machine makers, and mechanical engineers). It also changed the traditional job of the millwright.
As James F. Hobart wrote in his book, Millwrighting, "The ancient type of millwright has passed away. He has gone with the old time carpenter and obsolete shoemaker - the former with 500 pounds of molding planes and woodworking tools, the latter with nothing but pegging and sewing awls, hammer, and knife..." 
Modern millwrights work with steel and other materials and must often combine the skills of other mechanical trades in order to successfully install industrial machinery or to assemble machines from pre-fabricated parts. Modern millwrights must also be able to read blueprints and other schematics to aid them in the construction of complex systems. Millwrights are frequently unionized, with estimated numbers of around 45% in the US.
Millwrights in South Africa
In South Africa, the Millwright trade -especially focused on the mining sector- enjoys a far more versatile description than in other countries. At most Trades Training Centers, prospective Millwright Artisans are required to have a certain level of theoretical certification (e.g. N2 Certificate) and psychometric characteristics judged by thorough testing in order to qualify for the foundation practical & theoretical technical training. After being trained in a multitude of different fields (listed below), learner Millwrights enter in an apprenticeship for a specified time called "On the Job" Training. Here they work alongside all available artisans regardless of trade, depending on the institution.
Once they meet a structured quota of experience and pass the necessary “On the Job” tests, apprentices have two months to prepare for their practical Trade Test. Once passed, they receive the certification and status of a qualified Millwright Tradesman.
Fields wherein South African Millwrights may operate include: Low- Medium- and High Voltage Electrical fault finding, installations, and maintenance. PLC maintenance, installations, operations and fault finding. Fitting; installation and maintenance of most mechanical equipment, as well as Hydraulic and Pneumatic systems. Steelworks, rigging, and a few more fields all depending on the site, company, Mine Health and Safety Act, personal interest and specialization.
An example of a renowned Trades Training Center which entails the above-mentioned career path is Exxaro’s GROVOS Trades Training Center in Lephalale, Limpopo, ZA. This institution falls under the Exxaro Grootegeluk Coal mine in terms of the “On the Job” training.
Once Millwrights qualify, they have the opportunity to qualify as technicians, engineers, planners, foremen and many other routes requiring mainly electrical and mechanical expertise. Many Millwrights choose to enter the private sector to work on a contractual basis.
Oliver Evans was a prominent American millwright who published a manual for millwrights workers, etc.
A millwright today is someone who maintains, repairs or constructs industrial machinery for assembly lines, pumping stations and other utilities, print shops, steel mills, oil refineries, assembly plants, mining, power stations, saw mills, pulp mills and other, large or small, industries that are employing the use of, fixed, heavy machinery.
Millwrights are usually responsible for the unassembled equipment when it arrives at the job site. Using hoisting and moving equipment, they position the pieces that need to be assembled. Their job requires a thorough knowledge of the load-bearing capabilities of the equipment they use as well as an understanding of blueprints and technical instructions.
Millwrights must be able to read blueprints and schematic drawings to determine work procedures, to construct foundations for and to assemble, dismantle and overhaul machinery and equipment, using hand and power tools and to direct workers engaged in such endeavors. The use of lathes, milling machines and grinders may be required to make customized parts or repairs. In the course of work, millwrights are required to move, assemble and install machinery and equipment such as shafting, precision bearings, gear boxes, motors, mechanical clutches, conveyors, and tram rails, using hoists, pulleys, dollies, rollers, and trucks.
Millwrights are also involved in routine tasks, such as lubrication of machinery, bearing replacement, seal replacement, cleaning of parts during an overhaul and preventative maintenance.
Millwrights also must have a good understanding of fluid mechanics (hydraulics and pneumatics), and all of the components involved in these processes, such as valves, cylinders, pumps and compressors.
Modern standards of practice for millwrights also require working within precise limits or standards of accuracy, at heights without fear; the use of logical step-by-step procedures in work; planning, solving problems and decision-making based on quantifiable information.
Millwrights are trained to work with a wide array of precision tools, such as vernier calipers, micrometers, dial indicators, levels, gauge blocks, and optical and laser alignment tooling.
Areas of specialty
A typical job description for an industrial maintenance mechanic (millwright) often includes the primary purposes of installing, maintaining, upgrading and fabricating machinery and equipment according to layout plans, blueprints, and other drawings in industrial establishment.
Millwrights by nature of their profession have to be extremely well versed in many aspects of construction/demobilization. They may install a conveyor system at an airport one week and the following week work at an Industrial wastewater treatment plant.
Millwrights in the power generation industry can assemble, set, align and balance turbines/rotors, as well as install pumps, valves, cranes, fans, and travelling screens. Millwrights also perform critical lifts involving major components to be flown level at up to and within .005” (5 thousandths of an inch). Because of their training and expertise, Millwrights are generally chosen to work on tasks associated with flying and setting heavy machinery.
Millwrights are also in demand as teachers for vocational programs, both at the high school level and in post-secondary institutions. Many high schools feature fabrication courses that include metal work, where the experience of a qualified millwright is valuable. Often, these millwrights are paid a premium based on their years of field experience.
Most millwrights are educated through apprenticeship programs where they receive a combination of classroom education along with a good deal of on-the-job training. Most programs last about four years, and may include college degrees. Apprentices are usually paid a percentage of the average millwright's wage, and this percentage increases with experience.
Training courses received by a Union Millwright:
- Shaft alignment
- Field machining
- Electrical wiring
- Steel Fabrication
- Conveyor systems
- Steam Turbine installation
- Gas turbine installation
- Blueprint basic, intermediate and advanced
- Safety basic, intermediate and advanced
- Labor history
- Associates degree classes
- Evans, Oliver; Cadwallader Evans; Thomas Ellicott (1848). The young mill-wright and miller's guide, 12th edition. Lea & Blanchard.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
- Oleson 1984, pp. 325ff.; Oleson 2000, pp. 217–302; Donners & Waelkens 2002, pp. 10−15; Wikander 2000, pp. 371−400
- Adam Robert Lucas, 'Industrial Milling in the Ancient and Medieval Worlds. A Survey of the Evidence for an Industrial Revolution in Medieval Europe', Technology and Culture, Vol. 46, (Jan. 2005), pp. 1-30 (17).
- Prak, Maarten; an Luiten van Zanden; et al. (2013). Technology, Skills and the Pre-Modern Economy in the East and the West, Essays Dedicated to the Memory of S. R. Epstein. Brill. Explicit use of et al. in:
- Fairbairn, William (1863). Treatise on Mills and Millwork, Part I. London: Longmans, Green and Company. Retrieved 14 January 2016.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
- Hobart, James F. (James Francis) (1919). Millwrighting (2d ed., rev. and enl ed.). McGraw-Hill Book Company, inc.; [etc., etc.] Retrieved 14 January 2016.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>