A reaming tool for enlarging an underground borehole comprises a plurality of cutter assemblies (251, 252, 253) distributed azimuthally around a longitudinal axis of the tool, wherein each cutter assembly comprises support structure bearing a sequence of cutters (211-214) with leading surfaces facing in a direction of rotation of the tool and the sequence of cutters extends axially along the tool from an axial end (202) of the tool with the cutters positioned at radial distances from the tool axis which progressively increase as the sequence extends away from the axial end of the tool. The cutter assemblies include guiding structure (261-264) which is positioned circumferentially ahead of the leading faces of cutters of the sequence on the assembly. This guiding structure is configured such that the outline of the guiding structure is able to coincide with at least part of the notional surface described by the cutting outline of the preceding cutter assembly as the tool rotates, without any part of the guiding structure projecting outside that notional surface. This stabilises the positioning of the rotating tool in the borehole.