A walk-behind floor treatment machine comprising: a base portion (22,23) provided with and supported by at least one rotatable work head (27,28) for treating the floor, a handle portion for steering or guiding the machine along a working direction of travel and adapted to be pivotable with respect to the base portion, drive means (25,26) for rotating the work head or heads with respect to the base portion, floor-engaging wheel means for supporting the handle portion, the wheel means having a substantially transverse axis (Axis 2) of rotation so as to permit travel in the working direction, the wheel means being coupled to the base portion by a linkage which permits vertical travel of the base portion and associated work head or heads with respect to the wheel means, but which provides transverse constraint to limit or prevent yawing of the base portion with respect to the wheel means, wherein a lower region of the handle portion is pivotably connected to the wheel means via an articulated joint, wherein the pivoting is up / down pivoting of the handle portion about a transverse pivot axis (Axis 2), and wherein the articulated joint also permits side-to-side pivoting of the handle about a joint pivot axis (Axis 1, A-A') perpendicular to the transverse pivot axis, the arrangement being such that the handle portion may be manipulated to act on the wheel means so as to yaw steer the wheel means about a yaw axis defined by the floor-engaging contact of the wheel means, the yawing of the wheel means causing the base portion to yaw in response to yaw steering, and wherein a handle pivot lock mechanism is provided in which the adoption of a predetermined handle portion orientation permits one or more locking feature 17, 46, 47) to act on one or more corresponding constraint feature (44, 56, 57) so as to prevent pivoting movement of the handle portion with respect to the base portion. a base portion (22,23) provided with and supported by at least one rotatable work head (27,28) for treating the floor, a handle portion for steering or guiding the machine along a working direction of travel and adapted to be pivotable with respect to the base portion, drive means (25,26) for rotating the work head or heads with respect to the base portion, floor-engaging wheel means for supporting the handle portion, the wheel means having a substantially transverse axis (Axis 2) of rotation so as to permit travel in the working direction, the wheel means being coupled to the base portion by a linkage which permits vertical travel of the base portion and associated work head or heads with respect to the wheel means, but which provides transverse constraint to limit or prevent yawing of the base portion with respect to the wheel means, wherein a lower region of the handle portion is pivotably connected to the wheel means via an articulated joint, wherein the pivoting is up / down pivoting of the handle portion about a transverse pivot axis (Axis 2), and wherein the articulated joint also permits side-to-side pivoting of the handle about a joint pivot axis (Axis 1, A-A') perpendicular to the transverse pivot axis, the arrangement being such that the handle portion may be manipulated to act on the wheel means so as to yaw steer the wheel means about a yaw axis defined by the floor-engaging contact of the wheel means, the yawing of the wheel means causing the base portion to yaw in response to yaw steering, and wherein a handle pivot lock mechanism is provided in which the adoption of a predetermined handle portion orientation permits one or more locking feature 17, 46, 47) to act on one or more corresponding constraint feature (44, 56, 57) so as to prevent pivoting movement of the handle portion with respect to the base portion.