A combat engineer (also called pioneer or sapper) is a type of soldier who performs military engineering tasks in support of land forces combat operations. Combat engineers perform a variety of military engineering, tunnel and mine warfare tasks, as well as construction and demolition duties in and out of combat zones.

Combat engineers facilitate the mobility of friendly forces while impeding that of the enemy. They also work to assure the survivability of friendly forces, building fighting positions, fortifications, and roads. They conduct demolition missions and clear minefields manually or through use of specialized vehicles. Common combat engineer missions include construction and breaching of trenches, tank traps and other obstacles and fortifications; obstacle emplacement and bunker construction; route clearance and reconnaissance; bridge and road construction or destruction; emplacement and clearance of land mines; and combined arms breaching. Typically, combat engineers are also trained in infantry tactics and, when required, serve as provisional infantry.

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.

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 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.

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 sabotage, explosives demolition and tunnel warfare capabilities.

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.^{[citation needed]}