The 255th Tunnelling Company was one of the tunnelling companies of the Royal Engineers created by the British Army during World War I. The tunnelling units were occupied in offensive and defensive mining involving the placing and maintaining of mines under enemy lines, as well as other underground work such as the construction of deep dugouts for troop accommodation, the digging of subways, saps, cable trenches and underground chambers for signals and medical services.

The 173rd Tunnelling Company was one of the tunnelling companies of the Royal Engineers created by the British Army during World War I. The tunnelling units were occupied in offensive and defensive mining involving the placing and maintaining of mines under enemy lines, as well as other underground work such as the construction of deep dugouts for troop accommodation, the digging of subways, saps, cable trenches and underground chambers for signals and medical services. On 17 April 1915, 173rd Tunnelling Company became the first Royal Engineer tunnelling company to fire mines beneath enemy lines.