La munición trazadora (Trazadoras o trazadores) es un tipo especial de bala, modificada para aceptar una pequeña carga pirotécnica en su base. Esta se enciende al ser disparada, ardiendo intensamente y haciendo el proyectil visible para el ojo humano. Esto le permite al tirador seguir la trayectoria de la bala hacia el objetivo, permitiendo realizar correcciones en su precisión y puntería. Las balas de este tipo suelen estar formadas por un encamisado de cuproníquel y un núcleo de plomo con una cavidad en su base, en la que se aloja una sustancia compuesta por dos cargas: una iniciadora, compuesta de fósforo o silicio, y otra que produce la luminosidad, con un contenido de nitrato de estroncio o fósforo. Una vez producido el disparo, el fuego de la pólvora entra en contacto con la composición iniciadora, y ésta con la sustancia luminiscente, marcando el recorrido del proyectil hacia su objetivo. Existen diferentes tipos de balas trazadoras, algunas no empiezan a dejar la estela hasta transcurrido un cierto espacio (50 a 100 metros, según modelo) para que el enemigo no pueda localizar el lugar desde donde se ha disparado. Otras dejan una estela de humo, más visible durante el día. Y para operaciones especiales, las hay que dejan una estela sólo visible con visores nocturnos o de infrarrojos. Las trazadoras están intercaladas, comúnmente, a cada 4 o 6 cartuchos de munición ordinaria. El líder de pelotón, suele en ocasiones, cargar completamente su cargador con munición trazadora, para designar los objetivos a sus hombres. Para distingirlas del resto de cartuchos, la balas trazadoras suelen ir pintadas en la punta de color rojo o verde, u otro, dependiendo del país de utilización. Para quienes se encuentran en la trayectoria final de la munición trazadora, hay un efecto de ilusión óptica bien conocido, en donde los proyectiles parecieran viajar muy lento, pero mientras se aproximan, aumentan su velocidad considerablemente.

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Tracer ammunition (tracers) are bullets or cannon caliber projectiles that are built with a small pyrotechnic charge in their base. Ignited by the burning powder, the pyrotechnic composition burns very brightly, making the projectile trajectory visible to the naked eye during daylight, and very bright during nighttime firing. This enables the shooter to make aiming corrections without observing the impact of the rounds fired and without using the sights of the weapon. When used, tracers are usually loaded as every fifth round in machine gun belts, referred to as four-to-one tracer. Platoon and squad leaders will load some tracer rounds in their magazine or even use solely tracers to mark targets for their soldiers to fire on. Tracers are also sometimes placed two or three rounds from the bottom of magazines to alert the shooter that their weapon is almost empty. During World War II, aircraft with fixed machine guns or cannons mounted would sometimes have a series of tracer rounds added near the end of the ammunition belts, to alert the pilot that he was almost out of ammunition. More often, however, the entire magazine was loaded four-to-one, on both fixed offensive and flexible defensive guns, to help mitigate the difficulties of aerial gunnery. Tracers were very common on most WWII aircraft, with the exception of night fighters, which needed to be able to attack and shoot down the enemy before they realized they were under attack, and without betraying its own location to the enemy defensive gunners. The United States relied heavily on tracer ammunition for the defensive Browning M2 .50 caliber machine guns on their heavy bombers such as the B-24 Liberator, but they found that during deflection shooting, gunners who tried to aim using the tracers rather than the sights ended up not "leading" the enemy aircraft enough and missing, because of an optical illusion that made it appear that the tracers were striking the aircraft, when it reality they were passing significantly behind it. This proved true of fixed forward firing guns as well. In some cases, gunners were ordered not to use tracer ammunition at all, but in the end, with greater emphasis during training on using the sights and not trusting the tracers, the problem was solved to a large degree. Tracer rounds can also have a mild incendiary effect, and can ignite flammable substances on contact, provided the tracer compound has started burning and is still burning on impact.

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