A tunnel boring machine (TBM) also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre (done with micro-TBMs) to 19.25 m to date. Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs.

Tunnel boring machines are used as an alternative to drilling and blasting (D&B) methods in rock and conventional "hand mining" in soil. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct, and can be difficult to transport. However, as modern tunnels become longer, the cost of tunnel boring machines versus drill and blast is actually less—this is because tunneling with TBMs is much more efficient and results in a shorter project.

The largest diameter TBM, at 19.25 m, was built by Herrenknecht AG for a recent project in Orlovski Tunnel, St.Petersburg. The machine was built to bore through soft ground including sand and clay. Herrenknecht AG built also the world's largest diameter hard rock TBM: "Martina" (excavation diameter of 15.62 mt, total length 130 mt; excavation area of 192 square mt, thrust value 39,485 t, total weight 4,500 tons, total installed capacity 18 MW; yearly energy consumption about 62,000,000 kWh) is owned and operated by the Italian construction company Toto S.p.A. Costruzioni Generali (Toto Group) for the Sparvo gallery of the Italian Motorway Pass A1 ("Variante di Valico A1"), near Florence.

The former largest diameter hard rock TBM, at 14.4 m, was manufactured by The Robbins Company for Canada's Niagara Tunnel Project. The machine was used to bore a hydroelectric tunnel beneath Niagara Falls. The machine has been named "Big Becky" in reference to the Sir Adam Beck hydroelectric dams to which it is tunnelling to provide an additional hydroelectric tunnel.

In 2013, Hitachi Zosen Corporation delivered an earth pressure balance TBM to Seattle, United States for the Highway 99 tunnel project.[1] The bore is 57.5 ft (17.45 m).