Combat engineer
Combat engineer organization: Combat engineers play a key role in all armed forces of the world. They are invariably found closely integrated into the force structure of divisions, combat brigades, and smaller fighting units.

Combat support formations: In many countries, combat engineers provide combat support members of a broader military engineering corps or branch. Other nations have distinct combat engineering corps or branches; they are separate from other types of military engineers. The Danish military engineers' corps, for example, is almost entirely organized into one regiment of combat engineers, simply named Ingeniørregimentet ("The Engineering Regiment").

Combat arms formations: Combat engineer battalions are usually a part of a brigade combat team. During the War in Afghanistan and the 2003–2011 Iraq War, the U.S. Army tasked its combat engineers with route clearance missions designed to counter rising threats of improvised explosive devices (IEDs). To increase the effectiveness of these units, EOD and mechanic teams were typically embedded with the combat engineer platoon. Due to rising IED threats, the U.S. Army sends some combat engineers to complete Explosive Ordnance Clearance Agent training.

Special operations units: Individual combat engineers are often assigned as a component of both covert and overt direct action special operations teams. For example, the active duty US Army Special Forces and its two reserve components, the 19th SFG and 20th SFG of the US Army National Guard, employ combat engineer sergeants designated by MOS 18C. A Green Berets Operational Detachment Alpha, more commonly known as an "A-Team", typically consists of 12 men, two of whom are combat engineer sergeants.
Another example is the Israeli Yahalom unit, which is a special operations engineering unit, that possess the abilities of sabotage, demolition of explosives and tunnel warfare.

Terminology: A general combat engineer is often called a pioneer or sapper, terms derived respectively from the French and British armies. In some armies, pioneer and sapper indicate specific military ranks and levels of combat engineers, who work under fire in all seasons and may be allocated to different corps, as they were in the former Soviet Army, or they may be organized in the same corps. Geomatics (surveying and cartography) is another area of military engineering but is often performed by the combat engineers of some nations and in other cases is a separate responsibility, as was formerly the case in the Australian Army. While the officers of a combat engineer unit may be professionally certified civil or mechanical engineers, the non-commissioned members are generally not.

Sapper: In the British, Indian, Canadian, Australian, and New Zealand armies, a sapper is a soldier who has specialized combat engineer training. The term "sapper" in the U.S. Army refers to a person who either possesses the combat engineer military occupational specialty or who has graduated from the Sapper Leader Course, more commonly called "Sapper School." In Sapper School, volunteers from the ranks of combat engineers and other military occupational specialties (most of whom serve in the combat arms) undergo training in combat engineer and infantry battle drills, expedient demolitions, threat weapons, unarmed combat, mountaineering, and water operations. Some of the training in this 28-day course, arguably one of the most challenging in the U.S. Army, features covert infiltration techniques or survival skills.
In the Israeli Defense Forces, sapper (פלס) is a military profession code denoting a combat engineer who has graduated from various levels of combat engineering training. Sapper 05 is the basic level, Sapper 06 is the general level, Sapper 08 is the combat engineer commander's level, and Sapper 11 is the combat engineer officer level. All IDF sappers are also trained as Rifleman 07, matching infantry.
In the Canadian Army, it is a term for soldiers that have completed the basic Combat Engineer training.
In the Portuguese Army, a sapador de engenharia (engineering sapper) is a soldier of the engineering branch that has specialized combat engineer training. A sapador de infantaria (infantry sapper) is a soldier of the infantry branch that has a similar training and that usually serves in the combat support sapper platoon of an infantry battalion.
The Italian Army uses the term guastatori for their combat engineers.

Pioneer: In the Finnish army, pioneeri is the private equivalent rank in the army for a soldier who has completed the basic combat engineering training. Naval engineers retain the rank matruusi but bear the pioneeri insignia on their sleeves.
The German Bundeswehr uses the term Pionier for their combat engineers and other specialized units, who are associated with Special Forces to clear obstacles and perform engineering duties. Also the combat engineers in the Austro-Hungarian k.u.k. Forces were called "Pioniere".

Assault pioneer: In the British, Canadian, Australian and New Zealand armies, an assault pioneer is an infantry soldier with some limited combat engineer training in clearing obstacles during assaults and light engineering duties. Until recently, assault pioneers were responsible for the operation of flamethrowers.

Field engineer: Field engineer is a term used (or formerly used) in many Commonwealth armies. In modern usage, it is often synonymous with combat engineer. However, the term originally identified those military engineers who supported an army operating in the field instead of garrison engineers who built and supported permanent fixed bases. In its original usage, "field engineering" would have been inclusive of but broader than "combat engineering."

Specialisations: Sappers specialising in tunnel warfare may be known as miners.
In the French army, combat engineers specialising in bridge-building are pontoniers.

Practices and techniques: Combat engineers are force multipliers and enhance the survival of other troops through the use and practice of camouflage, reconnaissance, communications and other services. These include the construction of roads, bridges, field fortifications, obstacles and the construction and running of water points. In these roles, combat engineers use a wide variety of hand and power tools. They are also responsible for construction rigging, the use of explosives, and the carrying out of demolitions, obstacle clearance, and obstacle construction, assault of fortifications, use of assault boats in water obstacle crossings, helipad construction, general construction, route reconnaissance and road reconnaissance, and erecting communication installations. Combat engineers build and run water distribution points, carrying out water filtration, and NBC decontamination when necessary, and storage prior to distribution.
All these role activities and technologies are divided into several areas of combat engineering:

Mobility: Improving the ability of one's own force to move around the battlefield. Combat engineers typically support this role through reduction of enemy obstacles which include point and row minefields, anti-tank ditches, wire obstacles, concrete, and metal anti-vehicle barriers, and improvised explosive devices  (IED) and wall and door breaching in urban terrain. Mechanized combat engineer units also have armored vehicles capable of laying short bridges for limited gap-crossing.

Clearing terrain obstacles
Overcoming trenches and ditches
Opening routes for armored fighting vehicles
Constructing roads and bridges
Route clearance

Countermobility: Building obstacles to prevent the enemy from moving around the battlefield. Destroying bridges, blocking roads, creating airstrips, digging trenches, etc. Can also include planting land mines and anti-handling devices when authorized and directed to do so.
When the defender must retreat it is often desirable to destroy anything that may be of use to the enemy, particularly bridges, as their destruction can slow the advance of the attackers.

Planting land mines
Digging trenches and ditches
Demolishing roads and bridges

Explosive material handling: The placement of land mines to create minefields and their maintenance and removal.

Clearing fields of land mines
Demolition

Assault: Opening routes during an assault
Demolishing enemy structures (using bulldozers or explosive charges).

Defense structures: Building structures which enable one's own soldiers to survive on the battlefield. Examples include trenches, bunkers, shelters, and armored vehicle fighting positions.
Defensive fortifications are designed to prevent intrusion into the inner works by infantry. For minor defensive locations, these may only consist of simple walls and ditches. The design principle is to slow down the advance of attackers to where they can be destroyed by defenders from sheltered positions. Most large fortifications are not a single structure but rather a concentric series of fortifications of increasing strength.

Building fortifications
Building outposts
Building fences
Defense against WMD weapon threats

Equipment and vehicles: Combat engineers employ a wide range of transportation vehicles and equipment and use weapons unique to the engineers, including those used in land mine warfare.

Equipment used by combat engineers: Basic combat engineering tools include safe use of:

driving tools and chopping tools (hammers, mauls, sledges, screwdriver, and bits)
cutting tools and smoothing tools (saws, chisels, planes, files and rasps, brush-cutting tools, miscellaneous cutting tools)
drilling tools, boring tools, and countersinking tools
measuring tools, leveling tools and layout tools (rules, tapes, marking tools, levels, plumb bobs, squares)
gripping tools, prying tools, and twisting tools (pliers, wrenches, bars)
holding tools, raising tools, and grinding tools (vises, clamps, jacks, grinders, and oilstones)
timber handling tools and climbing tools; digging tools (shovels, posthole diggers, picks, and mattocks)
portable power tools and trailer-mounted tools (electric tool trailer and generator, portable power tools)
miscellaneous tools.