A computer aided surgery system, method, surgical method and instrument for guiding the positioning of an acetabular cup in a pelvis are described. The position of the transverse acetabular ligament of the pelvis and the position of the acetabular labrum of the pelvis are determined. The orientation of an acetabular plane including the acetabular labrum and the transverse acetabular ligament is calculated. A planned position for the acetabular cup is determined, in which an inlet plane of the cup is substantially parallel to the acetabular plane. The instrument comprises a mounting for attaching the instrument to a patient, a support, a guide defining a longitudinal axis, and for receiving a further instrument bearing the acetabular cup implant, and an adjustment mechanism. The adjustment mechanism can allow the angular orientation of the guide to be adjusted about at least two different axes to allow the inclination and version or anteversion of the longitudinal axis to be altered.