A whipstock comprises a longitudinal axis, a first end and a second end, and a tapering deflection section extending at least part of the way between the first end and the second end. The deflection section has a rear face. The deflection section has a front face, which in use will deflect a milling arrangement in a direction having a component which is perpendicular to the longitudinal axis of the whipstock. The front face has first and second side edges and a width defined between the first and second side edges. A depth of the deflection section is defined as a distance between the rear face and the front face. At at least one point along the length of the deflection surface, the depth of the deflection section at a distance from the first side edge is greater than the depth of the deflection section at the same distance from the second side edge.