In an in-line pipe inspection tool, sensors for inspecting the pipe are mounted on sensor blocks moveable relative to the body of the tool. However, when the sensor blocks move radially to conform to different pipe diameters, the circumferential distances between the sensors changes. To ameliorate the effect of this, the sensor blocks have a shape such that one axial edge of each sensor block circumferentially overlaps the opposite edge of an adjacent sensor block. With such an arrangement, when the sensor block are operating at minimum diameter, part of one sensor block will overlap an adjacent block, in the circumferential direction. As the diameter of the pipeline in which the pig is used increases, the degree of overlap will reduce, and may even reduce to zero, but there will still be no overall axial gaps between the sensor blocks. Thus, by suitable shaping of the sensor blocks the tool can be used with a wide range of pipe diameters.